52 THE BLACK HILL OF BARLSTON 



This puckering and convolution of the Silurian strata is 

 shared by the succeeding Lanarkian Rocks, and is, therefore, 

 of later date than these latter. On the other hand, the volcanic 

 rocks and the desert-formed sedimentary strata of the Cale- 

 donian Old Eed (the rocks which form St. Abb's Head, the 

 Cheviots, the Pentland Hills, etc.) lie undisturbed across the 

 ends of these convoluted Silurian and Lanarkian Eocks, and, 

 therefore, have clearly not participated in the movements 

 referred to. Hence, as before remarked, the disturbances in 

 question are entirely of older date than the Caledonian Old Red. 

 Furthermore, the last-named strata contain pebbles derived 

 from nearly all horizons in the Silurian (and older) rocks ; 

 and not only is that the case, but the Caledonian Old Red lies, 

 indifferently, upon the upturned ends of a vast thickness of 

 rocks, which range from the highest known, at one place, to the 

 lowest at another. Hence there must have been, also, prolonged 

 waste of the older strata prior to the deposition of the newer 

 series that lies unconforkiably upon their ends. Reference has 

 been made to this important fact on several previous occasions; 

 but it is well to repeat the statement here, in order that the 

 enormous extent of time implied by the gap between the 

 Silurian rocks on the one hand, and those which may surmount 

 them on the other, may be kept well in mind by the reader. 



There are many reasons for supposing that the Caledonian 

 Old Red formerly covered a large part of North Britain, 

 including the area under consideration. At Earlston these rocks 

 are now absent, because another period of denudation was 

 continued beyond the close of the continental conditions under 

 which the Caledonian Old Red was formed ; and, in conse- 

 quence, the last traces of that geological formation have long 

 since been swept away, and used again to form rocks of 

 subsequent date. The aggregate maximum thickness of the 

 Caledonian Old Red amounts to several miles, so that the lapse 

 of time implied by the unconformity under notice must be 

 prodigious. (See the report of the Exc&rsion to Oockburnspath.) 



The rocks which form the greater part of the lower portion- of 

 the Black Hill of Earlston belong to a much later geological 

 formation than the one referred to in the last paragraph. This 

 rock is usually red in colour, and also it commonly consists 

 of sandstone, either alone, or else mixed with conglomerate. 



