482 PKOi'. T. G. BONNET AND MAJOR-GEN. C. A. M'^MAHON 



We therefore think that the rocks along the coast, from the coast 

 east of Polpeor to Old Lizard Head, owe their peculiarities mainly to 

 subsequent mechanical disturbances, probably the result of an over- 

 thrust, so that the distinctive name had better be abandoned. The 

 coarsely crystalline gneissoid rocks discovered by Mr. Fox in the 

 outlying islands, and so admirably described by him and by Mr. 

 Teall, are situated, in our opinion, below the thrust plane, so that 

 we have here an association similar to that which occurs in some 

 parts of the N.W. Highlands of Scotland. At Porthalla, between 

 the great fault and the typical hornblende-schist (with serpentine), 

 a band of mica- schist occurs associated with fissile green schist. To 

 this mass we should apply a similar explanation, and no longer 

 desire to separate it from the Hornblendic Group *. 



lY. Igneous Kocks newer than the Serpentine. 

 (1) The Troctolite. 



This rock was described by Prof. Eonney under the name of 

 " the older gabbro," and its resemblance to the troctolite of Yol- 

 persdorf was pointed out. Subsequent analysis indicated that it 

 might be thus named, though it was not so typical an example. 

 It has also been described and figured by Mr. Teall T. Thus there 

 is little left to be said. It has been found only at Coverack Cove, 

 where it occurs both in irregular masses and in thin veins, with 

 little difference in the texture of the rock, and very perfectly welded 

 to the serpentine, which is practically unaffected by it. We cannot, 

 however, regard the association of the two rocks as a case either of 

 segregation or of veining strictly contemporaneous, for the ser- 

 pentine occasionally has been completely brecciated. For instance, 

 one block on the shore, which measured about 2' x 1|', consisted of 

 about equal parts of the two rocks, the serpentine being mostly in 

 rectangular pieces, the largest about 8" x 5", the smallest about 

 1^" X I", the thinnest vein of the troctolite being about 5" thick, 

 yet nearly as coarse as the rest. 



* Prof. Eonney is now convinced that Mr. Collins was right in regarding 

 the gneissoid band (described by him) as only a pressure-modified granite vein, 

 but both the Authors fail to understand on what grounds Mr. Collins separates 

 the hornblende-schists at Porthalla from those in other parts of the Lizard, and 

 considers them to be metamorphosed Lower-Silurian rocks. He states (Quart. 

 Journ. Ceol. Soc. vol. xl. (1884) p. 466) ' that the hornblende schist of Porthalla 

 is a very peculiar rock indeed.' We cannot understand how any one well 

 acquainted with the hornblende-schists of the Lizard could make this statement. 

 An exceptional specimen might be found anywhere, but speaking of the general 

 character of the Porthalla schist, which it must be remembered cannot be sepa- 

 rated from the mass which extends to Porthoustock Cove, we unhesitatinglyafiirm 

 that we cannot detect in it any valid distinction, macroscopic or microscopic, 

 from much of that which occurs in other parts of the Lizard peninsula. 



t ' British Petrogr.' pi. viii. fig. 2. 



