402 EECENT WOEK OF THE GEOLOGICAL SURVEY 



4. Succession in Loch-Broom district. 



Advancing southwards to the shores of Little Loch Broom, this 

 formation undergoes a still further development by the occurrence 

 of dark and grey flags and shales, occupying a higher position than 

 any of the beds lying to the north. These dark shales have also 

 yielded certain doubtful impressions which may prove to be organic, 

 together with worm-casts. In this region the total thickness of 

 strata is about 8000 feet. 



Considering the coarse materials which compose the greater 

 portion of the Cambrian strata and the indications of rapid 

 accumulation, it may be plausibly inferred that they represent a 

 great lacustrine formation. 



5, Formation of Outliers of Cambrian Strata in Post-Cambrian and 

 Pre-Silurian time by Folding and Denudation. 



After the accumulation of the Cambrian strata there is clear 

 evidence to prove that they must have been subjected to extensive 

 denudation prior to the deposition of the fossiliferous quartzites and 

 limestones. That there is a marked discordance between the two 

 formations, as originally established by Professor Nicol and Sir 

 Henry James, cannot for a moment be doubted. As the red sand- 

 stones are either horizontal or inclined to the E.S.E. at lower angles 

 than the quartzites, it is observable that the former are transgressed, 

 bed after bed, by the basal quartzites, till the latter rest directly on 

 the Archaean gneiss. This double unconformability is admirably 

 displayed on the slopes of Ben Garbh, forming the southern shore of 

 Loch Ass5rDt. 



That this was not the original eastern limit of the formation is 

 evident from the fact that several masses of Cambrian strata have been 

 carried from areas lying far to the east by the Post-Lower-Silurian 

 displacements to be referred to presently. The Cambrian age of 

 these thrust-masses is placed beyond doubt (1) by the occurrence of 

 the local conglomerate (" Button Stone "), (2) by the double 

 unconformability of the basal quartzites on the red sandstones and 

 the Archsean gneiss; and (3) by the sheets of intrusive felsite 

 occupying their proper horizon. 



It is obvious therefore that during the interval which elapsed 

 between the deposition of the Cambrian sandstones and Silurian 

 quartzites, the former must have been thrown into a series of gentle 

 folds, a vast thickness of strata was then removed, the Archaean 

 rocks were exposed over wide areas, and the surface was reduced to 

 a great plane of marine denudation. By these means various out- 

 liers of Cambrian strata were formed, far to the east of the present 

 apparent limit of this formation, which tell an interesting story in 

 connexion with the metamorphism induced by the Post-Lower- 

 Silurian movements (see fig. 6). 



