596 J. G. Goodchild — Deutozoic Rocks of North Britain. 



the beds below the lowest of these horizons do so or not cannot 

 be determined at present in the absence of any palseontological 

 evidence ; but there is a general belief amongst those who have 

 the means of forming an opinion that these lowest beds are, like 

 those above them, confined to the northern area referred to. 



To the south of the line just indicated there occurs another 

 formation, the general biological facies of whose fossils indicate, 

 as J. W. Salter pointed out nearly half a century ago, a lower 

 horizon than the Orcadian Old Eed. In places this formation, also, 

 is capped by the Upper Old Ked, which here, as elsewhere, lies 

 unconformably upon the rocks below. The formation to which 

 reference is now being made has been termed the Lower Old Red 

 Sandstone by several writers ; but as there is another and still older 

 formation which has also received that name, and as even the 

 Orcadian Old Red has been also called the Lower Old Red, it is 

 obviously better, in referring to these subdivisions, to make use of 

 some territorial name, in order to prevent ambiguity. Therefore, 

 many geologists who have felt the difficulty referred to have ado[ited 

 the name Caledonian for this southern Scottish formation. This, 

 besides being euphonious, and also useful in many wayss, is rendered 

 the more appropriate because the rocks denoted by that name were 

 considered by Sir Archibald Geikie to have been formed in a lake 

 to which he gave the name ' Lake Caledonia,' and furthermore, 

 because this formation is pre-eminently the Old Red of Scotland, 

 and because it is typically developed in the region distinguished by 

 the Romans as Caledonia. 



Like the Orcadian Old Red, the one under notice attains to a very 

 considerable thickness. Sir Archibald Geikie, indeed, estimates 

 that thickness at 20,000 feet (Text Book, vol. ii, p. 1008). Its 

 natural top is not seen ; nor, perhaps, is its base. As regards its 

 mode of origin, there appears to be evidence of a satisfactory nature 

 that the whole of this vast formation was accumulated under con- 

 tinental conditions, partly in large inland lakes, partly as torrential 

 deposits of various kinds, partly as old desert sands, and partly 

 as the results of extensive volcanic action. The lowest strata 

 appear to be those which are exposed near Dundee, where they 

 are brought up by a powerful anticlinal fold, whose effects are, 

 I think, augmented by a large fault which does not appear to 

 have been hitherto recognised. Most of the base on the north is 

 faulted in by the Higliland Boundary Fault. The newest strata 

 are exposed in the western part of the lowland tract of Strathmore, 

 along a powerful synclinal, which is correlative to the anticlinal just 

 referred to. 



Looking at the formation broadly, three subdivisions, founded 

 upon petrographical characters, can be made out. The highest of 

 these is formed by the conglomerates, sandstones, flags, and marls, 

 whose outcrops form the great lowland tract of Strathmore. The 

 middle subdivision consists chiefly of volcanic rocks, which are 

 mostly andesitic lavas, with some quite subordinate beds of tufi". 

 This volcanic series forms the Ochils and the Sidlaw Hills, iu the 



