390 C. CALLAWAY ON THE NEWER GNEISSIC 



with its associated " Upper " Quartzite, has thinned out in a little 

 over a mile ! 



After the preceding evidence of the overthrow of the Hebridean 

 from end to end of Assynt, I think I am justified in contending that 

 the mere superposition of one rock upon another is insufficient proof 

 of true conformity. At Knockan we have simply an overlie of the 

 gneiss along the strike ; and we have no opportunity of examining 

 the relations of the rock on the dip For this purpose we want a 

 valley or ravine cutting so deeply into the strata along the dip as to 

 expose the true anatomy of the district. Such an exposure is fur- 

 nished us by the next sections. 



Sections on the North and South Sides of Glen Coul (fig. 7). — 

 The region lying north of Loch Ailsh is so thickly covered that no 

 junctions are visible for several miles. In Glen Coul, however, the 

 conditions are very favourable for our purpose. The valley is cut 

 down so deeply as to expose the relations between the Hebridean, 

 the Assynt series, and the Caledonian, almost as in a diagram. 



At first sight, indeed, the phenomena appear to favour the 

 old view almost as strongly as the Knockan section. The 

 Stack of Glen Coul is formed of the newer gneiss ; and at its 

 western foot quartzite seems to pass conformably below it, the 

 two rocks being almost in contact. The ridge on the opposite 

 (north) side of the glen exhibits similar appearances. Here also 

 the newer gneiss forms a prominent peak; and in like manner 

 at its western base, the quartzite, followed however by flags and 

 dolomite, appears to dip below the gneiss. Following the junc- 

 tion for about a mile to the north, I found the same relations, except 

 that the flags and dolomite were wanting. 



Investigating the ground more closely, I ascertained new and 

 very striking facts. As we ascend the southern ridge, which leads 

 from the sea-loch up to the Stack, we pass over three exposures of 

 quartzite with Hebridean between. It might at first seem that the 

 quartzite was interstratified with the gneiss ; but this is certainly 

 not the case; for, on examining the slopes below, we find the quartzite 

 beds are not continued on the dip. Of some weight also is the fact 

 that both the lowermost and the uppermost of the three masses have 

 the ordinary seamy quartzite at the base. I therefore regard these 

 as faulted outliers of the quartzite on the loch to the west. 



The quartzite outlier in contact with the Caledonian is of course 

 the most important. Detailed study proved that the appearance of 

 conformity was only superficial ; for, while the Caledonian dips at 

 15°, the angle of the quartzite is 35°. The latter is of the annelidian 

 variety ; but the tabes are distorted. The rock is intensely squeezed 

 so that a coarse cleavage is produced, the particles of quartz are flat- 

 tened, and there is a mineral change, the aspect being distinctly that 

 of a partially metamorphosed rock (No. 98, p. 417). Following the 

 strata down the slope to the N.E., we find that, instead of passing 

 up into gneiss, the quartzite rises in dip, and is overlain by the 

 Seamy Quartzite (No. 97, p. 418), which becomes almost vertical, the 

 beds, however, curving over to the west. This quartzite has the 



