462 THE OLD RED SANDSTONE 



elevation. The reader will find this subject, and others connected with the 

 origin of the Old Red rocks in general, discussed at some length in the Pro- 

 ceedings of the Royal Physical "Society, vol. xiii., pp. 291-299. 



Considerable interest attaches to this Lanarkian Old Red now that an 

 abundant and varied ichthyic fauna has been discovered in them, which has 

 formed the subject of an important memoir by Dr. Traquair. 1 



I am inclined to think that the Lanarkian rocks were originally of con- 

 siderable thickness, and that, while the terrestrial oscillations to which they 

 are due were in progress, a band of marine limestone similar to the 

 Plymouth limestone was formed in connection with them. But upon this 

 point there is still some difference of opinion. 



This, however, is certain ; whatever strata may at one time have been 

 formed in connection with these rocks they soon afterwards underwent con- 

 siderable disturbance and denudation, which ended in the removal of nearly 

 the whole of the Lanarkian rocks, as well as a considerable thickness- 

 many thousand of feet in all probability of the older Silurian and Ordovician 

 rocks. Hence it has happened that only a few fragmentary patches of the 

 -strata originally formed have been left, and of these a still smaller part has 

 been exposed by later denudation. As a consequence, the Lanarkian rocks 

 are now only to be seen within limited areas. One of these lies in the heart 

 of the Pentland Hills, near the head-waters of the rivers Lyne and North 

 Esk. Another occurs on the flanks of Tinto. The third, and perhaps the 

 most important, occurs in connection with the Silurian outlier of the Logan 

 Water, near Lesmahagow, and near Muirkirk. 



The second division of the Old Red, in chronological order, is that great 

 group of strata which forms the Pentland Hills, the Cheviots, the Ochils, the 

 Sidlaws, etc. For this the name Lower Old Red Sandstone is still employed 

 by many geologists ; but having regard to the confusion likely to arise from 

 such a use of the term many others consider that it is as well to adopt some 

 more distinctive term, and they therefore refer to these rocks as the 

 Caledonian Old Red, seeing that it may be justly regarded as par excellence 

 THE Old Red of Scotland. 



This formation, in the area under notice, lies upon an exceptionally uneven 

 surface, which is shaped out of every rock older than the Caledonian Old 

 Red. In other words, there is an extensive and wide-spread unconformity, 

 the base of the Caledonian Old Red overstepping all the rocks older than 

 itself, from the highest surviving member of the Lanarkian Old Red down 

 to the lowest Ordovician rocks occurring in the district. Further, its middle, 

 or even its higher, subdivisions may overlap any one of the lower. 



Usually the basement beds consist of purplish- and chocolate-coloured 

 sandstones, which are well seen at the Falls of Clyde, and the area a few 

 miles to the south. Occasionally these are more or less conglomeratic, and 

 very commonly there is at the base a well-marked band of conglomerate 

 which is readily distinguished from the older conglomerate found in connec- 

 tion with the Lanarkian rocks by containing, amongst many other rocks, a 

 large percentage of grey wacke pebbles, derived .from the waste of the ancient 

 uplands of Silurian and Ordovician strata which formerly occupied the area 

 to the south. This greywacke-conglomerate has been found to be a very 

 useful datum by the officers of the Geological Survey who lately re-mapped 

 the district. 



As above remarked, the base of this formation is extremely variable as a 

 consequence, chiefly, of the unevenness of the old land-surface upon which it 

 accumulated. The irregularity is further increased by the fact that im- 

 portant volcanic outbreaks commenced at an early period in the history of 

 the formation, and were continued throughout a period of great length. 



1 Trans, S.SE. 3 vol. xxxix., part 3. 



