532 report — 1884. 



more commonly richly mottled red and green, freqaently containing 

 metallic bronzite. There is strong evidence that this rock is a hydrated 

 peridotite, and there is as clear and distinct proof of its intrusive character 

 as there is of any dolerite or felsite that I have ever seen. 



(/3) Troktolite : that is, a rock consisting mainly of anorthite (or an 

 allied felspar) and olivine, more or less changed into serpentine, with 

 but little of a pyroxenic constituent ; occurs only in one locality (Coverack 

 Cove). 



(y) Olivine-gabbro : in one great mass, and in numerous dykes and 

 veins on the east coast only. 



(8) Vein-granite : chiefly, if not entirely, on the west coast. 



(e) Diorites, hornblendic diabases, and basalts more or less altered ; 

 in small dykes and veins on the east coast. 



The chronological order of these rocks is that in which they are 

 enumerated, except that the granite is nowhere seen to cut the gabbro, 

 and it is impossible to fix the age of all the rather diverse members of 

 (e), though many of them are seen to cut both the serpentine and the 

 gabbro. As it happens, only one igneous rock — a kind of diabase — is 

 seen intrusive in the micaceous group (if the granitoid rock already 

 mentioned be of sedimentary origin) ; though, of course, as we may 

 presume it to underlie the later groups, it must be cut by the rocks which 

 intrude into them. 



Evidences of Geological Age. — The metamorphic rocks of the Lizard 

 peninsula are bounded on the north by a fault, which is exposed in 

 section on the west in the cliffs at Polurrian Cove, on the east at Porthalla 

 Cove. In the former we see very characteristic hornblende-schist in 

 apposition with a dark satiny slate, .parted by about a yard of fault 

 breccia ; the fault — as is so common in these cases — is a reversed one. At 

 Porthalla Cove the greenish schists of the micaceous group are faulted 

 against some filmy-looking mudstones. Here, as the two rocks have more 

 macroscopic resemblance and the rocks are much broken up by parallel 

 faults, it is more easy to imagine a transition ; but careful study with the 

 microscope will show that two rocks of very different characters are faulted 

 together. Further, at Nare Head, in the newer series, about a mile to 

 the north, is a conglomerate, containing, though rarely, fragments of the 

 true hornblende-schist and pebbles of a granitoid gneiss, not unlike a 

 vein-granite. This series cannot bs later than the Devonian period, and 

 is very probably rather older. ' 



(2.) Soxtih Devon (Start Point and Bolt Head). — The area occupied 

 by metamorphic rock is in greatest extent about nine miles from east to 

 west and two miles from north to south. Here also we are chiefly 

 dependent on the coast cliffs for our sections. In these are exposed two 

 kinds of rock — a lead-coloured mica-schist and a greenish chloritic rock 

 — varying from a moderately foliated to a rather massive rock, in the 

 lower part of which some thin bands of a rather dark mica-schist occur. 

 The former rock consists of quartz, generally rather inconspicuous macro- 

 scopically, and of three varieties of mica — a dark brown mica, a pale olive- 

 green mica (or possibly chlorite), and a silvery white mica, probably 



1 Full descriptions of the macroscopic and microscopic characters of the metamor- 

 phic and igneous rocks of the Lizard district are given in my papers, Quart. Journ. 

 Geol. Soc. vol. xxxiii. p. 884, and vol. xxxix. p. 1. Sir H. De la Beche's Geological 

 Report on Cornwall and Devon is still a great storehouse of most valuable 

 information. 



