Vol. 57.] CEUSH-CONGLOMEKATES OF ARGYLLSHIEE. 327 



deposits as to have been confounded with the Highland Bouider- 

 bed. Moreover, this occurrence of both at the same general 

 horizon may result in their appearance in the same section. In 

 doubtful cases, the only safe test of original deposition is the 

 presence of foreign fragments sufficiently large to escape being 

 confounded with the materials which form the matrix. Similarly, 

 the most satisfactory proof of deformational structure is the inclu- 

 sion of igneous boulders derived from rocks posterior in age to the 

 matrix. 



I have shown that in this district the only Dalradian igneous rocks 

 that occur are of intrusive origin, and that fragments of these enter 

 into the material of the crush-conglomerates, fixing the origin of the 

 latter beyond question. Owing to these phenomena not having 

 been understood, igneous rocks in a decomposed condition have been 

 considered as sedimentary. The junction of the epidiorites with 

 limestone is especially favourable for the production of these pseudo- 

 conglomerates. In this area the limestones of the crush-con- 

 glomerates are gritty, but this may be accidental, and normal 

 limestones might have displayed equal plasticity. 



These breccias were formed subsequent to the consolidation of the 

 igneous rocks, as they only occur in association with those portions 

 of the igneous masses that have undergone intense plication, of 

 which they express the ultimate result. 



The very nature of these structures tends to mask their identity : 

 the more they approach in character the appearance of normal con- 

 glomerates, the more completely is their history obliterated. It is 

 only a fortunate coincidence that has enabled me to demonstrate 

 the origin of these crush- conglomerates of the Highlands by such 

 an infallible test as that depending on the posterior age of their 

 included boulders. 



