448 



MR. B. nOBSON ON THE IGNEOUS HOCKS 



In microscopic section the rock shows hornblende, brown by 

 transmitted lig:ht, pleochroic in green and brown tints, in great 

 abundance in idiomorphic crystals. The rest of the rock appears to 

 consist of allotriomorphic felspar, quartz, leucoxene, iron pyrites, 

 and (subject to correction) abundant secondary epidote and some 

 calcite. I take the rock to be a diorite. 



There is some similarity in mineral composition between this rock 

 and that which I have described as a diabase [521] from Gullet 

 Buigh, although hornblende is very abundant in 250 and scarce in 

 521. Possibly further investigation may show them to be different 

 Tock fades assumed by the same original magma. 



Section 252. — About one mile west of Port Soderick is a farm- 

 house called Oatland, and nearly 370 yards W.S.W. of the farmhouse 

 an igneous rock is exposed in a small quarry in some uncultivated 

 ground. 



The rock is fine-grained and not dark- coloured, the colour being 

 mottled green and white. With a lens small idiomorphic white 

 felspar-crystals are readily made out ; quartz is abundant, and the 

 green constituent is an altered mica. 8p. grav. 2-72. 



In microscopic section small, cloudy, much altered, but very idio- 

 morphic felspars are abundant. They vary in size from 4 millim. 

 X 1'7 millim. to '64 millim. x '21 millim., and appear to be chiefly 

 plagioclase. Quartz is abundant and allotriomorphic, forming the 

 matrix in which the other constituents are embedded, and it frequently 

 exhibits undulose extinction. The altered mica is green, with dis- 

 tinct pleochroism, light green for rays vibrating parallel with the 

 cleavage-cracks, light straw-yellow for those at right angles to that 

 direction. This agrees with the pleochroism of chlorite. 



Associated with the altered mica is secondary epidote, varying 

 from brown to yellowish green in colour and exhibiting the green 

 interference- colour of the third order, while the quart/ shows the 

 yellow of the first order. The rock is a granite (in the wide sense), 

 and is quite distinct from that of Poxdale. 



Sections 203 and 2388. — Between Poortown and Rockmount, in 

 a quarry on the north side of the road 260 yards east of the Poor- 

 town picrite-porphyrite quarry, a large mass of igneous rock is 

 exposed. The rock is dark grey with greenish flecks, and is compact, 

 breaking with a clean, sharp, splintery fracture. Specific gravity 

 2-80. 



Much-altered porphyritic felspars are embedded in a groundmass 

 of felspar-laths and iron ore, the latter mostly changed to leucoxene. 

 The spaces between the felspar-laths of the groundmass are occupied 

 by green alteration-products. 



This rock bears a considerable resemblance microscopically to 

 the augite-porphyrite [500 a] at Scarlet Point. From an examina- 

 tion of a microscopic section kindly lent to me by Mr. Dickson I 

 believe this to be the same rock described by Messrs. Dickson and 

 Holland* as "specimen of gabbro from the most westerly quarry 



* Proc. L'pool Geol. See. vol. vi. pt. i. (1889) p. 128. 



