324 MR. J. B. HILL ON THE [Aug. I9OI, 



hornstone. It has been folded, fractured, and the fragments isolated, 

 the various stages of crush-conglomerate manufacture being so 

 apparent as to leave no doubt that we are dealing with a process of 

 mechanical deformation. 



Similar structures have also been detected within the epidiorites 

 themselves, although, owing to the comparatively homogeneous nature 

 of the material, they are not so conspicuous, and consequently may 

 easily escape observation. Pragments, however, of epidiorite may 

 here and there be seen enclosed in the main mass, as well as the 

 remains of crests and limbs of folds which have been torn from their 

 original position and augen-structures set up. 



Now that these phenomena are properly understood, other in- 

 stances in which boulders have been found embedded in a matrix 

 of material either similar to, or different from, the boulders can be 

 explained. Isolated boulders of quartzite have often been observed 

 in a matrix of the same material, likewise boulders of slate and of 

 limestone have been met with enclosed in limestone. For want of 

 a better explanation, they have been assumed to be related to the 

 Highland Boulder- bed, but there can be very little doubt that in this 

 part of Argyllshire they are more often crush- conglomerates, and 

 that structures formerlj^ supposed to be of sedimentary 

 origin are in many instances products of mechanical 

 deformation. 



ly. General Remarks. 



In discussing in the Summar)'^ of Progress of the Geological Survey 

 for 1899 ^ the crush-conglomerates of Cornwall, stress was laid on the 

 fact that the composite banded nature of the slates in which they occur 

 played an important part in the manufacture of those structures : 

 bands of alternating and divergent material offering a resistance to 

 stress less uniform than would be the case with more or less homo- 

 geneous masses. In Argyllshire we find likewise that crush- 

 conglomerates are most commonly produced near the junction of 

 rocks totally dissimilar in character, the junction of epidiorite and 

 limestone being especially favourable for their production. In this 

 respect the fact that the limestone associated with these crush- 

 conglomerates is generally of a gritty nature may be worth taking 

 into consideration. It is often thickly strewn with grains of quartz 

 and felspar, sometimes attaining even an inch in length, and it is 

 possible that the composite nature of this rock has tended to favour 

 differential movements within its mass, and rendered it easier to 

 accommodate itself to the tendencj^ to brecciation at the interfolding 

 of its junction with the tougher epidiorites. In this role it seems 

 to play so subordinate a part that, instead of contributing its own 

 share of boulders to the crush-conglomerate, its plasticity has been 

 such that it has been readily squeezed between the brecciated 

 epidiorite, and forms the matrix in which these boulders lie. It is 

 possible that a rock of more homogeneous composition would have 

 4)ffered sufficient resistance to prevent brecciation, and that instead 



^ See also Trans. Roy. Geol. Soc. Cornw. vol. xii, pt. vi (1901) p. 403. 



