123 



It appears then to be established beyond all doubt that the Lizard mass 

 of ultra-basic rock, from which the serpentine has been produced, varied 

 extremely in mineralogical composition : dunites, saxonites, Iherzolites, and 

 probably pit-rites were represented within it, and also olivine-hornblende 

 rocks. Taking- it as a whole, and leaving out of account the banded olivine- 

 hornblende rocks, it must have been very similar in character and composition 

 to the complex mass of Tertiary peridotites recently described by Professor 

 Jum> (1) as forming so large a portion of the Island of Rum. The Lizard 

 mass has, however, been both serpentinised and modified by regional 

 metamorphism, whereas, the core of the Tertiary volcano has been simply 

 exposed by denudation. To the north of the main mass of Lizard serpentine 

 are two small exposures of the same rock near Porthalla. Here it also occurs 

 in association with hornblende- schist. Speaking of the exposure nearest 

 Porthalla, Professor BONNEY says, (2) " The serpentine varies from a dull 

 purplish green to a greenish grey the latter colour denoting the more 

 weathered specimens. Sometimes it is a homogeneous dark rock, at other 

 times it exhibits a very marked streaky aspect, reminding one of the fluidal 

 structure of rhyolites. I consider the rock to be an altered peridotite, but to 

 have been originally rather finely crystalline. No crystals of enstatite, horn- 

 blende, or augite are now recognisable." Professor BONNEY regards the rock as 

 intrusive in the hornblende-schist. Mr. Collins gives a different interpreta- 

 tion. He considers that the two rocks are interbanded and connected with 

 each other by transitional forms. 



A small patch of serpentine occurs also near Nare Point, on the east of 

 Gerran's Bay. The relations of this mass are not well exposed. The 

 neighbouring rocks are gabbro, quartzite and other altered sediments. The 

 serpentine is in part a true massive serpentine with chrysotile veins, and in 

 part a soft serpentine-schist. Flat lenticles of the latter rock are 

 abundantly strewn over the lower slopes of a field at the back of the little bay 

 of Gilberick. This exposure of serpentine and gabbro is of some interest, 

 because it is known that the quartzite and its associated beds, including the 

 limestone of Gorran Haven, in which Mr. PKACH discovered fossils, is of 

 Ordoviciaii age. The relation of the igneous to the sedimentary beds is not 

 clear, but in the absence of pebbles of diallagc and serpentine from the 

 Ordoviciaii conglomerates of the neighbourhood, it seems probable that the 

 former were intrusive in the latter. If so it is evident from the serpentine- 

 schist that powerful earth movements have affected the district since the date 

 of intrusion. 



The following analyses represent the compositions of different varieties of 

 Lizard serpentine. 



(1) Q.J.G.S. Vol. XLL, 1885, p. 354. 



(2) Q.J.GS. Vol. XXXIX., 1883, p. 21. 



