374 C. CALLAWAY ON IHE NEWEK GNEISSIC 



the Assynt series should thin out towards the east is what might be 

 expected from the well-known behaviour of its lowest member, the 

 Torridon Sandstone. As regards the Dolomite, its frequent thinness 

 or absence is, I believe, due rather to faulting than deposit-con- 

 ditions. 



Dolomite of the dark-grey type is also seen a little above Annelidian 

 Quartzite near Rhie Cnoc, a mile or so south of Glen Coul. I did not 

 detect the Flags on a hasty inspection ; but if present, they could not 

 be of average thickness. 



Those who deny that the section in Glen Coul is merely the 

 ordinary Assynt group repeated to the east, must believe that, after 

 an interval represented by the " Logan Rock," the operations of 

 nature reproduced an almost exact facsimile of the " Lower " series 

 — viz. seamy quartzite with red felspar bits, annelidian quartzite, 

 brown flags, including two well-marked types, and dark dolo- 

 mite. I may safely assert that the science of geology has not 

 hitherto furnished us with an example of such a coincidence. 



B. The " Logan Rock " not separable from the Hebridean. 



This remarkable group attains considerable dimensions in Assynt. 

 Commencing in the south, we find it forming the mountain Scounan 

 (2028 feet). Then crossing the valley of the Oykel, it rises into 

 the Eagle Bock (2345 feet) (No. 76, p. 415); and it is continued to the 

 north in the Ben More group, constituting the whole of that extensive 

 mass, except a capping of grit and quartzite. It is enveloped 

 in quartzite in Ben Uarran ; but it reemerges at the western end ; 

 and being again covered in by the quartzite of Cnoc-an-drein, it 

 ascends to over 2000 feet in Glasven. From this point it is con- 

 tinued to the north, in elevations of from 1500 to 1700 feet, 

 across Lochs Glen Coul and Glen Dhu to Craig Dhu (No. 80, p. 416) 

 on Loch More. To the east of Glasven, it is represented in Dr. 

 Heddle's map as forming the main part of Ben Uie (Uidhe) and 

 the hills above Gorm Loch. 



This rock, whatever its age, is as much a gneiss as the Hebridean. 

 There is the same massive bedding, the same coarse crj^stallization, 

 the same toughness under the hammer, the same non-fissile fracture, 

 the presence of similar nodular masses of greenish hornblende, the 

 same abundance of epidosite *, and the prevalence of the same 

 essential minerals, quartz, felspar, and hornblende or mica. Such 

 differences as occur are merely varietal. " Logan Bock " within a 

 mile of Hebridean differs no more from it than do varieties of He- 

 bridean at the same distance fro™ each other. 



a. Objections to the Hebridean age of the "Logan Rock'' 

 considered. 



Dr. Heddle, comparing the lithology of the two rocks, affirms 

 that accessory minerals are present in the Hebridean, but wanting 



* Admitted by Prof. Heddle, according to Mr. Hudleston (Geol. Mag. Sept. 

 1882, p. 396). 



