OF THE CLYDE AREA. 463 



commonly one of the ancient lava-streams may be found to end off 

 suddenly, and sandstones, or even conglomerates, may be found side by side 

 with it. ' This fact has to be borne in mind in considering the geology of the 

 country around Tinto, and also many other areas in the Pentland Hills and 

 elsewhere. On the southeastern flank of Tinto the Caledonian Old Red is 

 represented by a series of andesitic lavas which overstep the various members 

 of the Lanarkian rocks in a manner which impresses one very much with the 

 importance of the chronological break between the two formations. On the 

 opposite side of Tinto we find the basement beds to be simply conglomerates 

 and sandstones, with not a trace of any volcanic rocks amongst them. It is 

 the same elsewhere. 



The general character of the Caledonian Old Red is that of a formation 

 accumulated under continental conditions, when a certain degree of aridity 

 characterised the climate. The torrential deposits from the ancient hills to 

 the south were spread out on the lowlands and in the shallow waters of 

 inland lakes, liable to be periodically dried up, wholly or in part. A chain 

 of volcanos contributed occasional streams of lava, mostly of andesitic com- 

 position, and now and then explosive eruptions took place, from which beds 

 of tuff, usually of 110 great thickness, were spread out far and wide over the 

 adjacent land". The volcanos must have attained to considerable dimensions 

 if one may judge by the magnitude of the remnants left. During the active 

 phases of volcanic action some considerable masses of intrusive rock were 

 left at different levels below the surface ; while, at great depths below the 

 volcano, the slow cooling, under enormous pressure, of the last of the volcanic 

 magmas gave rise to several interesting masses of granite, whose dominant 

 petrographical varieties may be termed diorite-granites, but which, as might 

 be expected, locally graduated into more basic, or more acid, varieties. 

 Considerable thermometamorphism arose in connection with these intrusions, 

 .as niav be seen in many places in the southwestern parts of the basin of the 

 Clyde." 



It is difficult, from the very fragmentary and disconnected nature of the 

 existing remnants of the Caledonian Old Red, to form any reliable estimate 

 of what its full thickness may have been. It is certain, however, that the 

 maximum in any part of the basin of the Clyde must have been considerably 

 less than has usually been considered to be the case. 



As regards the life of the period when the Caledonian Old Red was being 

 formed the evidence is scanty, and has to be sought chiefly outside the area. 

 It is recorded, however, that the late Mr. Brown of Lanfine once found a 

 specimen of Cephalaspis Lyelli in the sandstones near that place. Most of 

 the other fossils recorded from these rocks have been obtained from areas 

 beyond the- limit of that specially under notice. 



Remains of the Caledonian Old Red occur at several places within the 

 basin of the Clyde. They appear in force near the foot of Loch Lomond, 

 where they form the western end of the great tract constituting the Sidlaws, 

 the Ochils, and Strathmore. Discontinuous patches are exposed at several 

 localities between Lanark and Girvan, and others more extensive are, 

 doubtless, concealed beneath the unconformable covering formed by the 

 newer rocks. A third exposure is to be found in the northern half of the 

 Island of Arran. 



The andesitic lavas of these rocks form some of the chief repositories of the 

 Agates or " Scotch Pebbles," for which the land has so long been famous. 



The highest group of strata classed under the general name of the Old Red 

 Sandstone is the Upper Old Red. It consists almost entirely of sandstones 

 and conglomerates, which latter occur chiefly in its lower portion. It is 

 characterised, on the whole, by a much brighter-red coloration than either 

 of the others. In its upper part calcareous concretions or " cornstones " 

 occur. These mark the passage-beds into the Carboniferous rocks. They 



