472 PROF. T. G. BOXNEY AND MAJOR-GEN. C. A. M'^MAHON 



These intercalated pieces of schist cannot be traced far. Again, 

 near the eastern side of the serpentine, apparently enclosed in it, a 

 mass of regularl}' " bedded " hornblende-schist * may be seen at the 

 foot of the cliff. This is split by a wedge-like mass of serpentine, 

 which narrows down to less than a foot across, on the western side 

 of which comes a single ''bed" of schist about 5 inches thick. 

 Next comes a sheet of serpentine of about the same thickness, 

 followed by another block of hornblende-schist. In this, if we 

 mistake not, may be seen the end of another wedge of serpentine, 

 coming in the opposite direction to the former t. Again, with 

 regard to the asserted production of the serpentine from a stratified 

 rock, bearing more relation to the hornblende-schist than to a peri- 

 dotite, by some kind of transmutation not easy to understand, Mr. 

 Collins's own analyses J indicate that the serpentine varies from a 

 perfectly normal example of an altered peridotite to one which 

 exhibits some approach to the picrites §, a variation which is far 

 from rare. 



It is true that near the junctions the serpentine sometimes 

 becomes streaky or even somewhat banded in structure ; the horn- 

 blende-schist also departs a little from the normal type, as at some 

 other junctions, and is covered with a thin film of a steatitic 

 mineral. Thus the two rocks occasionally so closely resemble one 

 another that by the unaided eye alone they can hardly be distin- 

 guished ; the difference is, however, quickly perceived by continued 

 scratching with a knife, or by a few blows with the hammer, and is 

 obvious on microscopic examination. In other cases, however, the 

 two rocks are so clearly distinguished that the point of a knife may 

 be placed on their junction-plane ; while the changes in the horn- 

 blende-schist, which we pause for a moment to describe, are sug- 

 gestive here, as elsewhere, of contact-metamorphism. 



The hornblende-schist, in the localities where we examined it 

 in contact with the serpentine, is considerably altered. Macro- 

 scopically, it assumes a rather grey and slightly '* dusty " aspect ; 

 microscopically, it exhibits marked changes. "VYe have examined 

 slices from specimens obtained in three localities. 



(1) Specimen in contact with serpentine at Porthalla. This 

 rock exhibits a banded structure, and consists mainly of a colourless 

 micaceous mineral, the flakes varying in size in different bands, 

 which, with crossed nicols, gives low tints (white to milky grey) and 

 straight extinction, and of hornblende, which, in form and structure, 

 either resembles the last-named mineral or is somewhat acicular, 

 the hue varying from a pale brown to almost colourless. The 



^ The ' beds ' are almost vertical, and strike between W.N.W. and N.W. 



t The serpentine is rather rotten ; the section is, in places, obscured by 

 shingle, &c. ; and part of the hornblende-schist is stained of a reddish colour, so 

 that it resembles the serpentine. The mass of the former on the one side 

 of the first 'wedge' cannot exceed about four yards, and on the other about 

 three from this distance brings us again to serpentine. 



I See tables of analyses, facing p. 466. 



§ The term is used, not for a variety of the true peridotites, but for one in 

 which some felspar is commonly present. 



