178 J. A. PHILLIPS ON THE SO-CALLED 



a. Gabbros or dolerites, in which the originally constituent 

 minerals are occasionally, to a great extent, unchanged, but are 

 sometimes almost entirely represented by pseudomorphic forms. 



b. " Killas," or ordinary clay- slates. 



c. Highly basic hornblendic rocks, which exhibit a tendency to 

 break into thin plates ; these, under the microscope, present 

 the characteristic appearance of metamorphic slates. 



In addition to the foregoing, there are various slaty rocks of a 

 character intermediate between 6 and c, as in the village of Newlyn, 

 and on the western side of the valley north of Tolcarn. 



It follows that these rocks have been more correctly described by 

 some of the earlier geologists than by the more recent observers, who 

 have regarded them either as being exclusively " greenstones," or as 

 consisting of metamorphosed slates only. 



The rocks of the Cape-Cornwall district, coloured as greenstones 

 on the Geological Map, are principally hornblendic slates, which some- 

 times enclose veins or bands of garnet, magnetite, or axinite. Minute 

 garnets are not unfrequently disseminated throughout these slates. 



The rocks in the neighbourhood of the Gurnard's Head are almost 

 identical in character with those on the shores of Mount's Bay ; but 

 the slates are generally darker in colour, and are perhaps somewhat 

 more crystalline in structure, while the gabbros or dolerites have 

 sometimes been subjected to a still higher degree of metamorphism. 



The crystalline pyroxenic rocks and metamorphic slates of the 

 St.-Ives district exactly resemble those in the neighbourhood of 

 Penzance. 



The greenstones exposed between St. Erth and St. Stephens, un- 

 like the hornblendic and augitic rocks of the other districts, do not 

 occur in the immediate vicinity of granite ; but elvan courses are 

 invariably found at no considerable distance from them. The struc- 

 ture of these rocks would indicate that they are probably altered 

 ash- beds or hardened hornblendic slates, frequently containing 

 pyrites and minute garnets, with perhaps axinite and some other 

 minerals. Neither augite nor diallage was detected in any of the 

 sections examined. 



It is worthy of remark, that the chemical composition of these 

 slaty greenstones, or blue elvans, does not materially differ from 

 that of the gabbros or dolerites in the neighbourhood of Penzance, 

 the percentage of silica in the two series being as (will be observed) 

 nearly constant. 



The hornblende slates contain about ten per cent, less silica than 

 the crystalline pyroxenic rocks ; and there is an excess of iron 

 oxides present to nearly the same extent ; in other respects their 

 composition is very similar. 



Apatite is to be detected by the microscope in thin sections of the 

 augitic rocks only ; but phosphoric anhydride is to a small extent 

 present in rocks belonging to each of the several classes. 



The killas is a highly siliceous rock essentially differing from the 

 foregoing in chemical composition. 



