Monthly Archives: December 2015

Hunt the elvans

Before this series of post’s about the geological map of the Liskeard area launches into the ‘main event’, of the lodes it will have a pause at feature with the best sounding name, Elvan.

Elvans are intrusions of a quartz porphyry rock; the word is used for both the intrusion and the rock itself. Elvan is a very hard rock that can add greatly to the cost of driving levels and sinking shafts. Elvan patches near lodes were also thought to be associated with increased mineral richness. Its hardness made it a sought after building material.

Brenton Symons in his 1863 map shows elvans as red colour washed areas. He has shown eight setts in Caradon Hill area with elvan patches, as opposed to the British Gelogical Survey’s moderm map not indicating any.

No Elvan patches are shown at all by BGS.

“There  are  also  several  strong  elvans  or  dykes  generally  running  about  parallel  with  the lodes,  and  which  so  far  as  seen,  have  had  a  beneficial  effect.”

Webb and Geach

West Sharp Tor

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There  is  a  very  large  elvan  course,  north  of,  and  adjoining  the  lode;  the  thickness  of  which  though  driven  into  6  fathoms,  is  not  yet  known.

East Phoenix

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There  is  a  beautiful  white  elvan,  50  fathoms  east  of  the engine  shaft,  about  3  fathoms  wide.

Wheal Pollard

At  the  11 an  elvan  course  came  in  and  heaved  the  lode  south.

The  elvan,  of  which there  are  two  channels,  is  harder  than  general  in  the  district.”

Wheal Norris

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Caradon Consols

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East Wheal Agar

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“There  is  a  beautiful  white regular  elvan  course  70  fathoms  south  of  the  shaft,  whilst  the  Junction  of killas  and  granite  occurs  between  it  and  Dunsford’s  Shaft”

East Caradon

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“There  is  a  large  elvan  course  north  of and  accompanying  the  caunter,  and  in  the  same  elvan  a  little  north,  occurs Fawcett’s  Lode,  which  has  been  opened  on  for  some  distance:”

Wheal Caradon
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“There  are two  elvan  courses;  one  occupying  the  whole  space  between  the  north  and middle  lodes,  and  the  other  in  the  south  portion  of  the  sett;  white  porphyritic dykes almost destitute of mica, composed chiefly of feldspar and quartz.”

Wheal Hooper
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“It should be  noticed  that  two  elvan  courses  of  felspatic granite run parallel to the lodes.”

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The Geology of Caradon Hill- Cross-courses

This post continues the exploration of  the geology of Brenton Symons’ Victorian map of the Liskeard mining district’ with a wander around the cross-courses of the Caradon Hill area. My last post in the series came up with the conclusion that the modern British Geological map was more accurate than the 18th century publication, but a lot less pleasing on the eye. This post will put the two maps head to head on the topic of cross-courses.

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What is a cross course?
A cross course is a mineral vein running a near right angles to the predominant lode direction in an area. Cross-courses are normally non metallic but sometimes will carry lead and silver.

In Cornwall Cross-courses normally run in a north to south direction, apart from the in the St. Just area. Where a cross-course intersects a lode it throws it off its regular course; a break in the lode’s course called a ‘heave’.

Cross-courses gave mixed fortunes to a mine. Sometimes the heave would cause difficulties in tracing lodes,  they also could form a route for water to follow, a route that increased the risk of flooding workings.  On the plus side, cross-courses gave a line of softer rock for miners to follow, a weakness exploited to drive adits and cross-cuts.

Brenton Symons and Cross-Courses
Brenton Symons’ map denotes cross-courses by thick light grey lines. Their presence can also be detected by heaves in the lodes.  This post describes the location of the  cross-courses in the Caradon Hill area, and compares it with the information given in the contemporary Webb and Geach book,  and a modern BGS map. The cross-courses have been named by the mine setts through which they pass.

“There  are  several  cross-courses  running  through  the  country,  both  in  Killas  and  granite,  and  which  are found  as  is  usually  the  case,  to  influence  the  deposits  of  ore  wherever they  intersect  the  lodes.” Webb and Geach

The Cross-Courses

Wheal Pollard-Wheal Norris, Caradon Hill (Vale)

This long cross-course runs close to the main engine shafts of all three mines. Symons shows it becoming indistinct for a portion of its southern section, where he has assumed its course.

The British Geological Survey mao does not show the cross-course at all. The modern map does show some faults forming part of contact,

Webb and Geach mention the large cross-course in Wheal Norris and Caradon Hill mines. The latter mine used its weakness to drive an adit. Smaller cross-courses are mentioned in the book, but are not shown on the map.

Wheal Norris
This  sett  is  in  the  parish  of  St.  Cleer,  and  adjoins  Craddock  Moor  Mine,  having  the  same  lodes  traversing  it  for  500  fathoms  in  length.  There  are  in the  sett  nine  discovered  lodes,  which  are  at  right  angles  crossed  by  one  large cross-course  and  three  smaller  ones,  against  which  the  lodes  generally  make poor,  and  are  disarranged.

On  the  cross-course  directly  west  of  Carter’s  Shaft  a  cross-cut  has  been driven  north  40  fathoms,  intersecting  at  the  adit  level  two  large  masterly lodes

Caradon Hill
This  adit  has  been  driven  on  the  great  cross-course,  which  is  30  feet  wide, and  five  promising  lodes  have  been  cut,  producing  tin  and  copper;  it  is  still being  driven,  and  it  is  intended  to  proceed  with  it  through  the  entire  width  of the  sett,  with  a  view  to  cut  other  lodes  which  are  known  to  exist.

Craddock Moor-West Caradon
This cross-course cuts across the southeast corner of Craddock Moor’s sett, where Fox’s shaft is sunk on its course. No heave is evident on the lodes.

Yet again the BGS do not denote the existence of the cross-course. And yet again Webb and Geach describe more cross-courses than those shown by Symons.

Craddock Moor
There are  five  cross-courses  known  to  intersect  the  lodes,  three  of  which  are  from Caradon  Consols  which  is  immediately  south.

Gonomena-West Caradon

A consistent heave is shown by Brenton Symons; all the lodes western portions are displaced northwards. The BGS do not show the feature. Brenton Symons names it has the ‘West Caradon cross-course’ and the ‘Great Cross-course’ , ad states the feature was used to work the mine, and that it gave a heave if 2 to 6 fathoms. As in the previous cross-courses it is apparent that the  Victorian map only displayed the most significant cross courses.

Gonomena
The  boundary  cross-course  is  in  the  eastern  ground,  and  has  a  left-hand heave  displacing  the  lode  about  seven  fathoms.  West  Caradon  cross-course runs  through  the  centre  of  the  sett,  causing  a  right  hand-heave  of  about  three fathoms.  In  the  western  ground  three  other  cross-veins  come  in  from  West  Caradon and Craddock Moor, but these have not yet been seen in the mine.

West Caradon
These  lodes  are intersected  at  right  angles  by  numerous  cross-courses,  one  or  two  being  of a  large  size,  heaving  the  lodes  to  the  right  from  2  to  6  fathoms.  The  great cross-course  which  runs  through  the  centre  of  the  mine,  has  been  of  the  up most  service  in  working  the  mine  both  quickly  and  economically,  the  crosscuts  driven  on  its  course  costing  from  about  50s.  to  60s.  per  fathom,  which would  otherwise  have  to  be  driven  through  the  hard  granite,  at  a  cost  of  £12 or £14 per fathom.

South Caradon

This is Y shaped cross-course runs up the eastern slope of the Seaton valley, passing close to Jope’s shaft, and through Sump Shaft. The split of the Y is close north of Sump Shaft. The Cross-course causes the lodes western portions to be heaved norhwards.

This is the only cross course shown by the British Gelogicalk Survey. BGS show a single Cross-course running on the east of the Seaton River. It enters Gonomena set where its is heaved by a lode and then follows the western side of the openworks for a short distance. This coincides with the South Caradon Y cross course and a part of the great cross course. The gap in between the cross-courses shown by Symons coincides roughly with the heave shown by BGS

South Caradon
It will be seen that the whole of the Caradon lodes traverse  the  sett,  bearing  about  8°  north  of  west.  These  are  intersected  at  right angles  by  several  cross-courses,  the  easternmost,  near  Jope’s  Shaft,  heaving all  the  lodes  to  the  right  hand  regularly

The Great Cross-course

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South Caradon-Gonomena-South Phoenix-Phoenix
This cross course extends across a large portion of the map, from South Caradon to Phoenix. Its southern section, where it is called the boundary cross-course, is not directly shown by Brenton Symons. It can be identified however, by a heave in the lodes beneath the Seaton River.

This cross-course is shown only by the BGS in its southern section as it passes through the South Caradon and Gonamena Setts.

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South Caradon
Greenhill  Lode  is  driven  west  at  the  125  to  the cross-course about fathoms in length, 85  and contains  green  carbonate, grey  ore,  and  rich  oxide  of copper;  the  154  and  166 are  being  driven  to  get  under  this  ore,  in  the  confident  expectation  of  making  large  returns.  One  of  the  great  objects  of  the  adventurers  is  the  driving  of  the  126  cross-cut  south  on  the  great  cross-course, to  cut  Rosedown  and  Marke  Valley  lodes.

Phoenix
in a  deep  valley  streamed  for  tin,  is  a  large  cross-course  –  a  continuation,  in  fact, of  the  West  Caradon  boundary  cross-course,  which  there,  as  well  as  in  South Phoenix,  heaves  the  lodes  to  the  left  hand  about  10  fathoms.  This  crosscourse  is  many  fathoms  wide,  but  has  never  been  seen  at  the  Phoenix  Mines; as,  although  they  have  driven  on  a  course  of  ore  close  to  it,  they  were  afraid to  proceed,  on  account  of  the  probable  great  influx  of  water  that  would  ensue.

Wheal Hooper-South Caradon

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This cross-course may hold a clue to a mystery engine house. Its northern termination is close to the location of an isolated chimney whose purpose is not known. Symons shows and engine house located on the cross-course, a location that suggests that a shaft may have been sunk there in order to serve a cross-cut driven on the cross-course. This possible explanation for the mystery chimney is supported by the fact that the cross-course proved to be of great use to Wheal Hooper.

This is another cross-course is not shown by BGS.

Wheal Hooper
A cross-course,  which  has  been  of  considerable  utility  in  working  the  mine inexpensively,  stretches  across  the  sett,  bearing  a  few  degrees  west  of  north, and  intersecting  the  lodes  obliquely.  It  is  of  inconsiderable  magnitude,  and does  not  appear  to  affect  the  lodes  to  any  great  extent. WG

Glasgow Caradon

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Brenton Symons shows three cross courses, each one associated with shafts or adits.  The eastern cross-course has a heave of the western parts of the lodes to the north. This heave is similar to that shown on other cross-courses, that is the ground on the western side in moved northwards, or the eastern southwards.

Non of the Glasgow Caradon lodes are shown by BGS.

“He  drove  an  adit  south  on  a  large  crosscourse  about  four  feet  in  width,  and  cut  several  lodes.”

“A  cross-cut  from  the adit  has  been  driven  north  on  a  cross-course,  in  which  two  lodes  with  a  north underlie  have  been  cut,  containing  kindly  looking  gossan.

Marke Valley Consols
The Victorian map appears to show a short cross-course running northwards from one of the lode, close west of the dressing floors. Webb and Geach describes a cross-course that crosses all the lodes, a description that does not coincide with the map details.
No cross-courses are shown by BGS.

“South of  these  are  three  known  lodes,  one  of  which,  named  New  Lode,  has  been worked  to  the  80;  the  other  two  have  been  nearly  intersected  by  the  crosscourse  which  crosses  the  lodes  at  right  angles,  and  is  a  little  west  of  the  old whim shaft.”

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1 Wheal Pollard-Wheal Norris, Caradon Hill (Vale)
2 Craddock Moor-West Caradon
3 Gonomena-West Caradon
4 The Great Cross-course
5 South Caradon
6 Wheal Hooper-South Caradon
7 Glasgow Caradon
8 Marke Valley Console

Brenton Symons shows more cross-courses then the BGS, but less then those listed by webb and Geach.
Where a cross-course causes a heave, the ground to the west is northwards.
The cross-courses have been utilised by many of the mines for driving cross-cuts or adits.

Brenton Symons V British Geological Survey

Brenton Symons has the upper hand for this one. The Victorian map shows six cross-courses, whilst the BGS only one. The details described within the pages of Webb and Geach give credibility to the cross-course positions shown by Symons.  Therefore when it comes to cross-courses the Brenton Symons map is far superior to its modern counterpart.

Brenton Symons 1- BGS 1, Now a draw. Next round will be the elvans. 

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A brief pause from geology-The Really Handy Guide to the ISM Code is finished

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This blog normally is based on my writings on Maps, Mines, Engineers and Cornwall. But, hidden away in another part of the WordPress world is my blog on my other stream of writing, the sea.

Those books are written for seafarers studying for their ‘tickets’. They are study aids written to provide an affordable book alternative to the many excellent, but hugely expensive text books produced by the main publishers.

So, in the obscure off-chance that you love pouring over old maps, poking around mine remains, peering down dark Cornish holes and yet also earn your living by driving ships around the world here is a plug for my latest Kindle publication. A Really Handy Guide to the ISM code- A revision guide for mariners. 

If you do not have a clue what on earth the ISM code is about, then the book will not be for you. If that is the case , then perhaps you can justify opening this post by trying to spot the surfing dolphins in the cover picture.

ClIck here for my Author’s page on Amazon

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Bomin Moor Granite- Was Brenton Symons right?

In the previous posts I identified that the Liskeard 1863 Geological map of the Caradon Hill area and the modern British Geological Survey disagreed on the location of the granite/killas boundary. So which one was right?

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I hoped it was Brenton Symons Victorian map, for no other reason that it appeared more detailed. However, with a bit of digging in some of the books on the shelves I managed to discover which map was best.

Much of the modern evidence came from my well worn, and annotated copy of ‘Dines’, or to give its full title ‘The Metalliferous Mining Region of South West England’

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South Caradon Mine
Strangely enough, Dines was of not much help here

“Granite overlayed with killas in the south east.”

Was the only scrap of information it offered. Webb and Geach in their 1863 book was of more use though

“The  junction  of  killas  with  granite  occurs  a  little  south  of  Caunter and Kitto’s lodes. “

This statement coincides with the course of the contact shown on the modern map. It is rather strange that Symons was in error within such a well mined sett as South Caradon, and even stranger that he was in disagreement with Webb and Geach, considering their book and his map were believed to be have been produced in association with each other.

The Victorian authors support the modern location of the granite boundary within their reports on South Caradon

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Wheal Hooper
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Wheal Hooper
The  whole  of  the  sett  is  in killas,  which  overlays  the  granite  at  about  an  angle  of  45°,  and  the  junction occurring  at  the  northern  boundary,  the  engine  shaft  at  the  54  comes  into granite,  its  contact  with  the  slate  being  well-defined,  no  decomposition  having  taken  place.  It  should  be  noticed  that  two  elvan  courses  of  felspatic granite run parallel to the lodes.

Agents report
The  winze  below  the  62  (a  most  promising  point)  would,  however,  have been  proceeded  with,  but  for  a  great  influx  of  water  during  the  last  3ft. sinking,  causing  a  great  advance  in  the  price,  and  rendering  it  necessary to  purchase  a  larger  lift  in  order  to  proceed  with  the  work.  It  was  therefore thought  more  advisable  to  suspend  it,  as  the  cutting  of  the  lode  at  the  90 would  probably  drain  off  all  the  water,  and  enable  us  to  sink  the  winze  at a  very  considerable  saving,  and  without  the  aid  of  a  lift.  Near  the  bottom of  the  winze  is  a  sort  of  slide  which  appears  to  have  heaved  the  lode  to the  south,  whence  flows  the  water.  The  granite  in  the  bottom  of  the  winze is  of  favourable  description,  and  the  cleavages  are  faced  with  copper  ore

So far then, my hopes for Brenton Symon’s work being more accurate had been proven incorrect. The next mine to be looked at was second most important one on the map, Phoenix United.

Webb and Geach state-

Phoenix.
“The  present  workings  are  in  granite,  but  a  tongue  of  killas  is  deposited  in  the south-eastern  portion  of  the  sett,  in  which  is  a  promising  lode  worked  on  the backs  for  a  long  distance,  and  called  the  Snuff-box  Lode.”

Dines presented an excellent resource to disentangle the complex geology here, nice cross section of workings on the main lode.

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Part of this diagram is shown here (Copyright BGS). The plan show the lode running west to east. My annotated yellow line is the granite/Killas contact. Killas to the right, granite to the left. It is indicated reaching the surface close east of West’s Shaft. The conclusion from this fact is that again the Modern map is more accurate.

The dotted yellow line is the Great cross-course, more on that feature in the next post.

The final mine I studied in detail was South Phoenix.  Brenton Symons shows that sett within granite, and yet the British Geological Survey clearly show a large slab of killas intruding between two faults as far west at the Hurlers.

Again Dines contained a diagram that provided an answer.

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This cross section runs north to south across the South Phoenix Sett. It clearly shows the ground between Prosper Shaft and Parson’s Shaft being ‘clay slate’. An indication that yet again the modern map is more accurate.

Despite of my desire to prove the superiority of the Victorian cartography, when it came to depicting the granite/killas contact the modern map was clearly superior.

British Geological Survey 1, Brenton Symons 0

Next round would be the cross-courses, or faults. 


 

Brenton Symons’ 1863 map is reproduced in ‘The Liskeard Mining Area in 1863’

Webb and Geach’s book is available in paperback.

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