NICOL SOUTHEETS GEAMPIANS. 191 



derived from these rocks; this, however, is not the case. I ex- 

 amined a very large number of specimens collected indiscriminately 

 in several localities, and found them, almost without exception, frag- 

 ments of igneous rocks, and especially of clove-brown felspar-por- 

 phyries and clay stone -porphyries. They are thus derived from rocks 

 distinct both from the augitic greenstone on which they rest, and 

 from the crystalline rocks forming the mountains on the north and 

 west. This peculiar character of the conglomerates of the Old Red 

 Sandstone in the central valley of Scotland is by no means confined 

 to this locality ; it reappears in many other places, both along the 

 foot of the Grampians and on the south side of the valley near the 

 Ochil Range. Thus, in the coarse conglomerates near the Bridge of 

 Allan, similar brown felspar-porphyries and claystones predominate, 

 in many places to the exclusion of almost every other rock. In 

 the Ochils, rocks of this nature, from which these conglomerates 

 may have been derived, are not uncommon ; but near Comrie they 

 either do not occur, or are probably concealed by newer deposits. 

 The whole phenomena of these red-sandstone-conglomerates lead to 

 the conclusion that they have been formed along lines of fissure, 

 through which outbursts of igneous matter were taking place, — one 

 of these lines running along the foot of the Grampians, and another 

 marked by the present Sidlaw, Ochil, and Campsie Hills. Along 

 these lines the materials of the conglomerates are chiefly trappean. 

 In some localities, however, as near Callander, fragments of quartz 

 or primary strata predominate, the rocks in these places having pro- 

 bably been formed near the outlet of some ancient river, or where 

 projecting points of the old strata rose high above the waters. and 

 were thus exposed to atmospheric and oceanic influences. It is also a 

 curious fact that, mixed with these trappean conglomerates, there are 

 other beds of finer materials, undoubtedly derived from the older cry- 

 stalline rocks. Thus, some of the Pteraspis-sandstones at the Bridge 

 of Allan consist of angular fragments of quartz, felspar, and mica, and 

 hence like a true granite or gneiss, only with a fragmentary or 

 clastic, not a crystalline structure. The prevailing red colour of the 

 sandstones also seems owing to the ejection of ferruginous matter 

 during these long- continued eruptions*. 



The next formation is the clay- slate and greywacke, which pro- 

 bably underlie the flat plain south of Comrie, though concealed by 

 the covering of detritus. They are, however, well seen on the north 

 side of the Earn, especially in the deep gorge of the Lednock, which 

 has cut through them in a series of romantic rapids and cascades. 

 The rocks vary from a moderately coarse greywacke, composed of 

 rounded grains of quartz in a basis of clay-slate, to a fine-grained 

 clay- slate with a fibrous or silky lustre. The greywackes are of a 



* Whilst these facts show that part of the igneous formations in the Ochils pre- 

 ceded, and that part accompanied the formation of the Old Red Sandstone, there 

 is full evidence that other portions are younger than the Red Sandstone, and indeed 

 even than the Coal. It is also right to mention that some observers consider the 

 trappean conglomerates as marking an older, the quartzose conglomerates a 

 newer stage in the Red Sandstone. (See Page's Advanced Text-book, p. 174.) 



