﻿part 2] 



THE TSLAY ANTICLINE. 



143 



low angles, and, as Mr. Wilkinson has pointed out, a cake of con- 

 glomerate catches on to Beannan Dubh, a little hill rising to the 

 east of Loch Lossit. 



The lower portion of the conglomerate is clearly an aqueous 

 deposit, showing obvious bedding, and split up by numerous inter- 

 stratified bands of quartzite and sandy cream-coloured dolomite. 

 The upper portion, including the main mass, is imbedded, and 

 carries its blocks, boulders, and pebbles promiscuously in a brown, 

 clayey and sandy, somewhat calcareous (likely dolomitic) matrix. 



The most conspicuous fragments in the deposit are nord- 

 markites, of unknown source, and pieces of cream - coloured 

 ■dolomite — like the dolomite intercalations, only purer. Mr. Wil- 

 kinson has described Islay-Limestone pebbles as a feature of the 

 conglomerate, but I spent three da} r s in searching without finding 

 one. It seems probable that his statement has originated on the 

 assumption, already mentioned, that certain conglomeratic lime- 

 stone-bands belong to the Portaskaig Conglomerate : whereas they 

 appear really to be an integral portion of the Islay Limestone 

 itself. There is no doubt, too, that the dolomite-fragments have 

 often been reckoned as limestone. 



Other rocks represented in the Portaskaig Conglomerate have 

 been compared by Mr. Wilkinson with the Lewisian and Torridonian 

 west of the Loch Skerrols Thrust. It is interesting to find gneisses 

 and grits among the boulders, but I think that one should hesitate 

 before assigning them to any particular source. 



The Beannan Dubh exposures furnish invaluable evidence in 

 determining the original order of superposition among the sedi- 

 mentary groups overlying the Loch Skerrols Thrust. The sandy 

 cream-coloured dolomites interstratified in the lower portion of the 

 conglomerate are very prominent in this section. They have been 

 described by Mr. Wilkinson, and their outcrop is indicated on the 

 Geological Survey 1-inch map, Sheet 27, though too small to 

 reproduce in PI. XII. A careful examination of these beds shows 

 that they have suffered £ contemporaneous erosion,' and have yielded 

 numerous fragments to the conglomerate ; and the rule seems to 

 be that the fragments of any particular bed of dolomite enrich 

 the immediately overlying bed of conglomerate. 



One of the dolomite-bands is especially noteworthy. It is of a 

 paler tint than usual, and is well exposed for a couple of hundred 

 yards along the south-eastern face of the hill. It rests with an even 

 base upon shales, but is of very irregular thickness, as if its upper 

 surface had suffered from erosion. The overlying rock is a brown, 

 gritty, well-bedded dolomite, which extends downwards into the 

 cavities chai'acterizing the top of the white dolomite below. 

 Moreover, the lower part of the brown dolomite usually contains 

 numerous large fragments of the underlying stratum — in fact, 

 there is often a fod't or two of coarse breccia between the two 

 layers, consisting of angular blocks of dolomite set in a sparse, 

 brown, gritty matrix. 



Now, this evidence, carefully considered in the field, left no 



m2 



