550 E. B. Tcmney — Woochvardian Laboratory Notes. 



Under the microscope the felspar is found to be all plagioclase fairly- 

 clear and fresh, the extinction angle on either side of the twin face 

 being small, points to their being oligoclase. The ground contains 

 some glassy base in interstices, but is made up chiefly of plagioclase 

 microlites with calcareous patches of decomposition, and a crowd of 

 magnetite particles, giving doubtless the dark colour to the rock. 

 Separate crystals of green hornblende are fairly numerous, and with 

 them are a few crystals of augite. Largish crystals of black oxide 

 of iron (ilmenite ?) are often connected with the hornblende ; apa- 

 tite, which is abundant, is usually in the same connexion. This is 

 a freshly-preserved porphyrite with andesitic structure. While the 

 rock at one side of the quarry is nearly black, at the other side may 

 be found a pale greenish variety in which are occasional segregations 

 of crystalline calcite up to an inch in diameter. It is, however, only 

 the same rock weathered. A slice off a specimen in the Sedgwick 

 Collection [P. 59] is to be referred here. Its felspars are nearly 

 entirely decomposed, the hornblende has disappeared, as have also 

 the small felspar prisms of the ground, their place being taken by 

 nuclei or patches of crystalline calcite showing cleavage and twin- 

 ning ; the ground is largely occupied by a greyish-brown granulated 

 material, which runs together into most involved forms ; under high 

 powers it is seen to be not opaque, but to polarize very feebly, pro- 

 bably several decomposition products enter into its constitution. 

 Very minute grains and nests of secondary quartz lie in its folds. 

 Apatite alone remains unchanged. 



Another example collected by me shows an intermediate stage of 

 decomposition, and serves to explain the great change wrought in the 

 arrangement of the rock by decomposition. It would have been 

 impossible to determine the extreme case unless it had been recog- 

 nized as coming from this quarry, in which all stages are found. 

 This porphyrite may be found to extend for some hundred yards in 

 breadth. Turning to other parts of the hill, we find it composed 

 mostly of a fine-grained grey rock, not so dark as the porphyrite 

 of the quarry just mentioned; it contains minute felspars; when 

 weathered, however, it becomes a brownish-grey, more or less speckled 

 with felspathic portions of pale ochrey tint. It may be recognized 

 as having to the eye the same characters all over the hill. Examples 

 of it from various points were submitted to microscopical analysis, 

 the specimens chosen being as little weathered as was possible to 

 obtain. 



Cam Boduan, from quarry on west side of hill by the encircling 

 wall. — This is a fine-grained rock, quite compact-looking, of a more 

 greenish-grey tint than usual. Under the microscope it is seen to 

 be much decomposed, the larger felspars almost entirely converted 

 to aggregates of variegatedly polarizing particles. The ground has 

 the same structure as that of the first-mentioned quarry, consisting 

 of microprisms of elongated form, their long axes here being often 

 parallel ; they are less decomposed than the larger ones, and ex- 

 tinguish in a way which proves their triclinic nature ; some of their 

 outlines are, however, merged into decomposing material, though 



