386 Geological Society. 



are chlorite slate, and a rock composed of alternate laminse of com- 

 pact white felspar and green mica ; in the hollow below is contorted 

 gneiss, connected intimately with the rock just described, or rather 

 passing into it. 



Compact white, and pale-green felspar occurs frequently in the 

 slates, at and near their junction with the granite. 



The granite at the sides of Ben -Nevis is large-grained, composed 

 of flesh-coloured felspar, albite, gray-quartz, and black mica in equal 

 proportions ; higher up, it loses the albite and quartz, acquires a few 

 specks of hornblende, and passes into a kind of felspar-porphyry j 

 which last-mentioned substance constitutes the summit. 



The junction of the granite and porphyry is laid bare on the E. and 

 S. sides of the mountain ; but on the N. and W. is concealed by 

 scattered blocks of porphyry. 



At the head of Glen Ptarmigan, is a steep cliff of porphyry, at least 

 1500 feet high. Its shape is that of an oblique four-sided pyramid, 

 irregular and truncated, rising on the east and south, through the 

 granite ; and not merely overlying it, as M. Boue supposed. This 

 fact the authors consider themselves as having fully established. 



With equal confidence they affirm, that the gneiss and mica-slate 

 are not conformable to the granite ; and that the latter has forced its 

 way through them : the granite traverses them also in the form of veins. 



They remark further, the frequent occurrence of compact felspar, 

 where these substances adjoin the granite. 



■2. The mountains N. of Ben-Nevis are chiefly mica-slate : S.E. of 

 Loch Lochy this rock passes into gneiss ; on tlie sides of Glen Gloy, 

 Glen Tuntick, and Glen Roy, it contains garnets, and alternates with 

 quartz rock ; in the valley of the Spean it is interstratified with gra- 

 nular limestone. 



Felspar, porpliyry, and greenstone occur, in the mica-slate, in Glen 

 Gloy, in Glen Rov, at Caldivan, and in the valley of the Spean. 



The S. shore of Glen-Nevis, near Ballahulish, is a granitic aggregate 

 of felspar and mica ; with concretions of mica and hornblende : granite 

 occupies the low ground; gneiss succeeds, passing eastward, into mica- 

 slate and clay-slate, in which are beds of roof-slate alternating with, 

 and traversed by, greenstone dykes, and interstratified with granular 

 limestone. 



In Glen Coe mica-slate is cut through obliquely by compact felspar- 

 porphyry ; in the bed of the river is a fine-grained granite, with con- 

 cretions like those of Ballahulish ; the granite is succeeded by gneiss 

 at a lower level, and at a higher, by compact felspar, speckled and 

 veined with epidote. 



3. On the Isle of Skv the authors offer the following observations : 



The syenite lies upon the hyperstene rock ; the passage into which 

 is not gradual, but abrupt ; the hyperstene rock passes into compact 

 greenstone, and often skirts the syenitic mountains; the lias rests on 

 syenite, or forms detached outliers ; and this observation holds good 

 invariably. 



There is no such thing as a vein of syenite in the lias. The trans- 

 mutation of lias into white granular and compact limestone is more 



constant 



