162 A. Somervail — Schists of the Lizard District. 



groups of Prof. Bonney, F.R.S., and Sir H. De la Becbe. Although 

 separated into this triple division, I quite agree with the Professor 

 that these " form but one series." I cannot, however, regard them 

 as forming a strictly ascending or descending series in the order 

 enumerated ; neither am I certain that one of the groups, the " talco- 

 micaceous," can any longer be regarded as a separate and independent 

 group in itself for reasons to follow. 



I shall briefly notice the leading petrological characters of each 

 group of rocks, referring the reader to the elaborate descriptions of 

 Prof. Bonney x and Mr. J. J. H. Teall, 2 M.A., F.G.S., for minute 

 information. 



I. The Granulitic Group. 



This group was founded by Prof. Bonney to receive certain rocks 

 held by him to form the uppermost of the three series, and composed 

 " of greyish granitoid rocks, composed mainly of quartz and felspar, 

 sometimes almost a quartzite, sometimes almost simulating a vein- 

 granite, associated with more hornblendic, chloritic, earthy and 

 micaceous layers." 



These " granulitic " rocks throughout the entire extent of the area 

 certainly form a very large variety, ranging from a rock scarcely 

 discernible from an ordinary granite, to dark compact diorites, the 

 latter frequently highly porphyritic. There are also quartzo-felspathic 

 vai'ieties in which one or other mineral predominates, sometimes 

 with the addition of hornblende in variable quantities. Other 

 varieties consist simply of pure felspar, or nearly so, from a granular, 

 to a fine homogeneous texture. These might be classified under 

 granites, granulites, tonalites, quartz- diorites, and diorites. Nearly 

 all of these varieties occur in more or less extensive or isolated 

 masses, and frequently pass by transition into each other. They 

 also occur in many areas of the district interbanded together, aptly 

 described by Mr. Teall as the " crystalline banded series." 



II. The Hornblendic Group. 



These rocks for the most part consist of dark hornblende schists 

 and more compact varieties of rock resembling diorites with a com- 

 plete transition between both. A porphyritic structure more or less 

 abounds in certain areas throughout both the rocks and schists. In 

 the latter, as would be inferred, this structure frequently exhibits 

 the effects of crushing. 



Besides these normal hornblendic rocks, which form the bulk of 

 this group, there are, however, many other varieties associated with 

 them, such as mica-schists, felsitic-like-rock, quartzose and quartzo- 

 felspathic varieties approaching the granulitic type. Although these 

 are certainly very subordinate, yet they must he taken into account 

 in dealing with the origin and relations of these rocks as a whole. 



Mr. Howard Fox, F.G.S., was, I believe, the first to discover 

 mica-schists (resembling those at Polpeor Cove) in the very heart 

 of the hornblende area at Pen Olver, Trecrobin and Polledan. 

 These mica-schists, which have been carefully measured and 



1 Q. J.G.S. vol. xxxiii. p. 884 ; vol. xxxix. p. 1. 2 . British Petrology. 



