VICI2«^ITT OF T£LE UPPER PAET OF LOCH MAEEE. 97 



Maree, as far as Ben Slioch, and a little to the north of the Loch- 

 Maree hotel*. They vary considerahly, some, as in the Laggau 

 valley, being mainly quartz and felspar, with a little of a green 

 micaceous mineral, others ordinary mica- or hornblende-gneiss, 

 others, again, rather hornblendic or chloritic schists. All, how- 

 ever, exhibit conspicuously that massive structure which early 

 observers rightly fixed upon as characteristic, and, as a rule, 

 are fairly uniform for a considerable vertical thickness. The 

 microscope shows that quartz and felspar are always present, exhi- 

 biting the rather irregular granular form characteristic of gneissic as 

 opposed to granitic rocks (fig. 4). The quartz is generally fairly clear. 



Fig. 4. — Hebridean Gneiss from near base of Ben Slioch, 



enclosures or cavities being verj^ minute, though in one or two cases 

 there is a considerable amount of dusty opacite. The felspar, usually 

 more or less decomposed, varies in quantity, rather predominating in 

 the specimens from near the junction in Glen Laggan. These appear 

 to contain microcline ; it is, perhaps, present in the others with con- 

 siderable quantities of plagioclase (probably oligoclase), as well as 

 orthoclase. Epidote microliths and other decomposition products are 

 present largely in some specimens. One of those from Glen Laggan 

 consists almost wholly of quartz and felspar, with a minute quantity of 

 a pale hornblendic (?) mineral, and a Kttle opacite and magnetite, 

 being almost exactly like specimens from North Wales and Shropshire, 

 for which I have proposed the name granitoidite. Another (more 

 gneissic) contains a chloritic mineral and a little pale-coloured mica. 

 A third, macroscopically much greener than the others, appears to 

 owe its colour chiefly to quantities of very minute epidote. 



Mica is conspicuously present in two specimens collected by the road- 

 side north of Loch-Maree hotel, in one from the floor of Glen Laggan, 

 some distance up stream above the gorge, and in one collected by 

 the shore of Loch Maree, at the south end of the base of Ben Slioch. 



* The third group and most of the second are universally admitted to be 

 the Hebridean gneiss. 



Q. J. G. S. No. 141. H 



