420 PROP. T. G. BONNEY ON A SERIES OF ROCKS 



quantity of hornblende. This occurs in fair-sized crystals, with well- 

 denned external form and cleavage, in small elongated prisms and 

 belonites, and in fibrous groups associated withaserpentinous mineral. 

 One or two of the first variety show zonal structure or enclose portions 

 of the matrix. The rock may be called a hornblendic porphyrite. 



72 (1J mile east of Altnagalagach, p. 409). This is a most perplex- 

 ing rock. In the slide a fair quantity of black mica is at once re- 

 cognized, and a number of subtranslucent sap-brown garnets, the 

 larger (being the less regularly formed) including flakes of mica &c. 

 I refer some colourless crystals to apatite. The ground of the slide 

 appears to consist partly of a felspar in patches of a most irregular 

 form (with, perhaps, a little quartz), and a mineral which occurs in 

 rather wavy bunches, like tufts of long thread or of rootlets, or a kind 

 of " canal system." It seems to have replaced the felspar, and may 

 be one of the fibrolite group. 



82 (Balloch) consists of a plagioclastic felspar much decomposed, 

 and of a green hornblende intercrystallized with quartz or in clus- 

 tered microliths. It may be a diorite ; but it has been so messed 

 by crushing &c, that it is impossible to be sure. If metamorphic, 

 it belongs to the Hebridean series. 



109 (Half-mile KW. Kinloch Ailsh, p. 409) consists mainly of fel- 

 spar associated with a little quartz, hornblende, sphene, andiron per- 

 oxide. The nature of the felspar is obscured by a kind of micro- 

 graphitic structure, or root-like intergrowth of quartz. I think I 

 recognize microcline. If igneous, the rock is a kind of syenite ; but 

 in its present condition it is difficult to be sure. If not igneous, 

 it belongs to the Hebridean series. 



Discussion. 



The President said that the district about Loch Assynt had 

 always appeared to him to be very difficult ; but it was impossible 

 not to admire the pains spent by Dr. Callaway on his paper. 



Mr. Httdleston said that the author's work evidently was very 

 careful and conscientious. He had himself examined the Assynt 

 country during part of last summer. In the " Assynt series " of the 

 author, the sequence was fairly clear. The Torridon Sandstone and 

 Ben-More grit, each in its own district, formed the base ; then came 

 the quartzite, the intermediate group, and finally the dolomite. He 

 thought that there was no " upper quartzite," but that the appearance 

 of it was due to repetition. What had produced that repetition ? 

 It was probably the Logan rock. Under that term he included the 

 great gneissoid series with local injections which had figured as the 

 "igneous" rock of former authors. In the mountains of Upper 

 Assynt this formed the core round which the quartzites &c. were 

 folded in great winding sheets. He was not quite prepared at pre- 

 sent to accept the absolute identity of the Logan rock with the 

 Hebridean gneiss ; there were important petrological differences, re- 

 cognized by men like Nicol and Heddle. The Logan rock attained 

 an elevation of 2500 feet on the south-east flank of Coniveall : so 



