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?

image

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’

image

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

image

Wheal Hooper
.

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.

image

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.

image

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.

Advertisements
Tagged , , , , , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: