Vol. 57.] CRUSH-CONGLOMERATES OF ARGYLLSHIRE. 325 



of the formation of a crush-conglomerate merely a folded junction 

 would have been produced, like that at Arichamish (pp. 318-19). 



The study of those crush-conglomerates has also brought out the 

 fact that not onl}^- are they largely confined to rocks similar in 

 lithological character, but they also occur mainly at a definite 

 horizon, which may to some extent account for their being regarded 

 as of sedimentary origin. It will be readily understood that con- 

 siderable difficulties may arise in determining whether such rocks 

 owe their structures to original deposition or to mechanical deform- 

 ation. Especially will this be the case when, as may easily happen, 

 the rock which has given rise to these superinduced structures was 

 itself originally a conglomerate. For instance, the gritty limestone at 

 Creag nam Pitheach, although not coarse enough in its structure 

 to be classed with a conglomerate, yet occasionally contains pebbles, 

 probably original, which are as large as some of the smallest frag- 

 ments formed by the crushing of the epidiorite. And it is evident 

 that where the same mass exhibits structures, some of which are 

 original and others sujDerinduced, caution is required in any de- 

 termination based on such evidence. In many instances great 

 difficulties must arise in distinguishing structures which are depo- 

 sitional from those produced by deformation : the most valuable 

 aid to such determination is the presence of foreign boulders. In 

 cases, however, where the matrix of the crush-conglomerate is 

 coarse, we may see pebbles of foreign material which are original 

 and depositional, approaching in size the smaller boulders due to 

 deformation ; and in a crush-conglomerate of this type the presence 

 of these pebbles might be construed as evidence that the whole 

 deposit was a boulder-bed. Caution must be used, therefore, to 

 make certain that the foreign boulders which are selected to decide 

 the question approximate in size to the boulders the origin of which 

 it is sought to determine. 



In the same way the crush-conglomerate may contain boulders 

 the source of which is not now visible in the immediate neighbour- 

 hood, and which would appear to favour therefore a depositional 

 origin. But the evidence of the Eilean-Liver section, in which 

 tiny strips of limestone are seen infolded with the big sill at a distance 

 of nearly a quarter of a mile from the nearest visible limestone, 

 demonstrates the fallacy of this reasoning. In so disturbed a region 

 we must consider whether the parent mass had not such an under- 

 ground or overhead extension, from which the crush -conglomerate 

 boulders might have been derived. As an illustration, the crush-con- 

 glomerate of Creag nam Fitheach may again be cited. Let us 

 suppose, for example, the top of this platform to be stripped by 

 denudation of its surface-cap. The basement of the epidiorite and 

 its junction with the limestone would then be removed, and we should 

 see on the north-western flank of the hill a limestone enclosing 

 well-rounded boulders of vesicular epidiorite having all the appear- 

 ance of a boulder-bed, and the nearest visible source of supply for 

 these boulders being now far removed from such deposit, its 

 supposed depositional character would be strengthened. 



