340 LIET/T.-GEN. C. A. M c MAHOK ON ROCKS OF [Aug. 1 894, 



The rocks of trie eastern line of outcrop, 2, 6, 7, and 8, exhibit 

 two sets of joints at right angles to each other, the principal one 

 dipping about N". 10° "W., the dip being steeper than the sides of the 

 hill. Both these sets of joints simulate bedding-planes, and the 

 apparent dip depends upon which of the two series of joints is 

 locally dominant. 



The western line of outcrop (3, 4, 5, 10, 11, and 12 of the map) 

 consists of porphyritic epidiorite, to be next described. 



I have examined under the microscope thin slices of specimens 

 from these outcrops as follows : — 



No. l l 1137. Sp, Gr.2-97. 



„ 2 1126. „ 2-92. 



„ 3 1213. „ 2-91. 



„ 4 1214. „ 286. 



„ 5 1133. „ 2-84. 



These are dark-grey rocks, with an almost compact matrix in 

 which numerous porphyritic, somewhat rounded crystals of white 

 felspar are embedded. They are all altered dolerites, and were 

 originally composed of augite and felspar. The remains of unaltered 

 augite may be still seen in four out of the five specimens, being 

 especially prominent in JNos. 2, 3, and 4. 



The augite has been altered into a light-green hornblende. Some- 

 times the unaltered core of augite, colourless and non-pleochroic, 

 with an extinction-angle of from 35° to 38°, remains in the centre of 

 a crystal, while the pale-green pleochroic hornblende, which sur- 

 rounds it, extinguishes at an angle of 5°. The original augite 

 played the part of groundmass, the felspar appearing as large 

 porphyritic crystals, and also in lath-shaped prisms penetrating 

 into, and embedded in, the pyroxene. The felspar is intensely 

 altered, having been converted either into a pale-green chloritic 

 substance or into a serpentinous-looking isotropic mineral. Ilmenite 

 is very abundant in all the slices ; two of them contain a little 

 mica ; two possess apatite ; and one of them a little granular calcite. 



There is nothing to show that these rocks contained olivine. 



Volcanic Tuffs. 



The following rocks, from the places marked 1, 2, 6, and 9 on the 

 accompanying map, have now to be described : — 



No. 6 1127. Sp. Gr. 2-80. 



„ 7 1134. „ 2-79. 



„ 8 1217. „ 2-77. 



„ 9 1140. „ 2-75. 



„ 10 1227. „ 2-74. 



„ 11 1128. „ 274. 



„ 12 1210. „ 273. 



„ 13 1212. „ 2-73. 



1 Nos. ] to 57 are the serial numbers of the specimens described in the paper, 

 and are given for the convenience of the reader. The higher numbers are 

 those marked on the hand-specimens, and refer to the author's English 

 collection. 



