342 1IEUI.-GEN. C. A. M c MAHON ON BOCKS OF [Aug. 1 894, 



pale yellowish -brown mica and in some cases with opalescent 

 quartz. The fragments are pierced and invaded by a cryptocrys- 

 talline magma, which contains a great quantity of anthophyllite, 

 reddish to yellowish-brown mica, and some quartz. 



The anthophyllite is in bundles and sheaves of fine, needle-like, 

 radiating prisms without crystallographic terminations. It is 

 colourless in thin sections and consequently not dichroic. It pola- 

 rizes in the yellow of the first order ; it has straight extinction ; the 

 major axis is at right angles to the length of the prism ; and it is 

 not decomposed by prolonged heating in hydrochloric or in sulphuric 

 acids. Its refraction is considerable, as it exhibits sharp and dark 

 outlines. In a cross section I obtained two cleavages meeting at 

 127°. All the facts above stated seem to indicate that the mineral 

 is anthophyllite : it is evidently a secondary product, and occurs 

 principally in the base, or matrix, but it is also to be found sparsely 

 in the included fragments, more particularly along their margins. 



The magma, or base, has frequently eaten its way into the in- 

 cluded fragments, these tongues sometimes terminating in a cul-de- 

 sac ; at other times pieces of the included fragments have been 

 detached, drawn out into strings, and included in the fluxion of the 

 magma. 



No. 9. The hand-specimen does not, on its fractured surface, 

 give any indication of being a clastic rock, but under the microscope 

 fragments of three distinct rocks can be made out ; one a highly 

 crystalline lava; another a compact, buff-coloured felsite ; and a third, 

 which may be an altered sedimentary rock or the micro-granular 

 base of a rhyolitic lava. A line of shear comes between fragments 

 of the 2nd and 3rd class, and eye-shaped patches of both rocks are 

 entangled with each other. 



No. 10 was from a loose block not in situ, as will be explained 

 farther on. Macroscopically considered, the hand-specimen is seen 

 to contain numerous, small, slaty-looking fragments. The microscope 

 reveals the presence of pieces of six or seven different lavas, all 

 extremely fine-grained, but differing from each other in colour and 

 structure. The interstitial cement (metamorphosed fine volcanic 

 dust that originally filled up the interstices between the fragments) 

 can be made out in parts of the slide ; but in other parts, its place 

 is taken by a rhyolite composed of two differently-coloured glasses, 

 and containing porphyritic crystals of quartz and felspar. This 

 rhyolite corrodes and invades the included fragments, and carries 

 off small pieces detached from them in its course. It indicates 

 the intrusion of a rhyolitic felsite into an ash. 



No. 11 . This is very much the same kind of rock as the last. 

 The fragments are of the same varieties of lava, and the intruding 

 rock is evidently the same as that seen in No. 10, only it has lost its 

 rhyolitic character somewhat and has passed into an ordinary felsite. 

 The rock has ceased to be an ash, and has become an igneous rock 

 full of included fragments. The fragments are not so numerous or 

 so closely packed as in No. 10, and probably indicate that the 

 rhyolitic felsite was beginning to get clear of the ash. 



