20 IGNEOUS ROCKS OF SOUTH-EAST CORNWAIiL. 



I believe that we may find our igneous rocks of very great 

 utility in tMs direction ; and 1 propose to illustrate my meaning 

 by some notes on recent inquiries into the character of the rocks 

 of South-East Cornwall. 



I must premise by pointing out that the help to be derived 

 from the existing Survey Map is limited. Numerous exposures, 

 and some of these among the most stratigraphically valuable, are 

 unmapped. Those that are mapped are coloured under four 

 classes only — granite, elvan, greenstone, and serpentine. It is 

 the vague third term that causes all our uncertainty, and compels 

 an examination in the field before we can decide what these 

 " greenstone " rocks really are. It should be understood that no 

 attempt is being made to depreciate Sir Henry de la Beche's 

 really marvellous work. What he did no one at that day could 

 have done better ; but it is little short of a national scandal that 

 after nearly fifty years the Geological Map of the most impor- 

 tant mining centre of the kingdom should remain practically as 

 he left it. 



There are in South-East Cornwall six classes of igneous 

 rocks, more or less distinctive in character : — 



1 . — Lavas (Dunstones) with Ashes and Tuffs. 2.— Grabbros. 

 3, — Dolerites. 4. — Serpentine or Picrite. 5. — Granites and 

 Elvans. 6. — Triassic Basalt or Felsite. The ages of three of 

 these we can fairly fix, and thus obtain some clue to that of the 

 rocks in which they occur. 



The Lavas are unquestionably Devonian. They are contem- 

 poraneous with, and interstratified among, the Devonian slates 

 that underlie the Plymouth limestone, ranging up to contact with 

 it. They are associated with ash-beds and tuffs, in the main 

 schistose, which often graduate into the slate in such a way that 

 it is impossible to say precisely where the one series ends and 

 the other begins. They are moreover not only Devonian, but 

 characteristic within our area of a definite section of the Devon- 

 ian period. With the containing rocks they have been much 

 disturbed, and in the vicinity of Plymouth and Saltash are thrown 

 into a series of folds with frequent repetition, much more 

 characteristic in the field than on the map — though there the 

 parallelism is striking enough. 



