388 Transactions. — Geology. 



occasionally on the pericline. The actinolitic hornblende 

 mentioned in B 102 is not present here, but the groups of 

 brown mica plates are rather more numerous. 



(B 103.) Gneiss. — A white rock consisting of an even- 

 grained mixture of quartz and feldspar, with a few plates of 

 biotite. In the hand specimen a gneissic structure can be 

 seen. Section : Quartz completely allotriomorphic ; is very 

 abundant. Feldspar generally twinned only on the Carlsbad 

 law, but sometimes oligoclase on the albite law and frequently 

 in smaller crystals, showing a further development of peri- 

 cline twinning, giving an appearance almost similar to that of 

 microcline. Slightly decomposed into grains that are for the 

 most part muscovite plates. Biotite in fairly large grains is 

 not infrequent ; it is evidently an original constituent. A few 

 grains of zircon and magnetite, and a little apatite. 



(B 107.) Ajjlite. — Hard compact rock, light- green. Se- 

 parate grains not distinct. Section : Quartz in irregular 

 clear grains, allotriomorphic. Feldspar much decomposed. 

 Decomposition products needles of a light-green colour, pro- 

 bably epidote, and plates apparently of muscovite. Optical 

 properties not entirely lost by decomposition, and much is 

 evidently orthoclase. Other crystals show albite and occa- 

 sionally pericline twinning. Extinction angle of lamellae 

 small, showing the mineral to be oligoclase. The only ferro- 

 maguesian constituents are very scarce ; biotite mica plates 

 partly converted into chlorite. The green colour of the 

 rock, which in hand specimens is conspicuous, is entirely 

 due to the epidotic decomposition products of the feldspar. 

 The specimen is penetrated by small microscopic quartz 

 veins. The specimen has sandstone adhering to it, and is 

 evidently a dyke rock. 



(A 188.) Syenite. — Very compact, showing dark crystals 

 of hornblende of even size arranged in a white feldspathic 

 material, which weathers on the surface into a light-green 

 tint. Section : Feldspar almost opaque owing to complete 

 decomposition. Products of decomposition tine colourless 

 needles, with low refractive index and very weak bire- 

 fringence, probably kaoliuite. No twinning can be detected 

 in the small patches of feldspar remaining clear, so it is 

 probably orthoclase. Hornblende much bleached, apparently 

 originally brown. Absorption, a, light-straw colour, (3, green- 

 ish-brown, y, brown, y > (3 > a. In the bleached portion 

 of the crystals there is barely any noticeable absorption. 

 Cleavage traces much bent and distorted in longitudinal 

 section. 



(B 106.) Syenite. — Generally similar in section and hand 

 specimen to A 188, but much finer-grained. It is evidently a 

 portion of the same mass. 



