38 MONOGRAPH OF DURA DEN. 



chapter of Siluria, after stating that tlie oldest known rocks of 

 the British Isles are composed of a highly crystalline, horn- 

 blendic gneiss, with powerful granite veins, and that in the 

 North Highlands of Scotland there is from that rock an ascend- 

 ing order in proceeding from north-west to south-east, proceeds 

 to observe " that the conglomerates and sandstones which in 

 the North-eastern Highlands form the base of this series, are 

 compounded out of all the pre-existing rocks above described, 

 whether gneiss, older grits, quartz rock, mica-schist, granite, 

 &c., as ranging in ascending order from the west to the east 

 coast. It is this great and thick lower zone of the Old Red of 

 the Northern Highlands which I consider to be equivalents in 

 time of those beds on the eastern and southern flanks of the 

 Grampians, — Arbroath, Dundee, and Balruddery, — and which, 

 unquestionably, constitute the base of the group, as demon- 

 strated in England by their conformable union downward with 

 the true Ludlow rocks or uppermost Silurian." 



" The true base," continues Sir Roderick, " in Shropshire and 

 Herefordshire of the old red sandstone, properly so called, is, I 

 repeat, seen to be a red rock, containing cephalaspis and pter- 

 aspis, and gradually passing down into the grey Ludlow rock ; 

 and in both of these contiguous and united strata, remains of 

 large pterygoti are found, but of different species in the two 

 bands. Now, although the Arbroath paving-stone, and the 

 grey rocks ranging to the north of Dundee, much resemble the 

 uppermost Ludlow rock, they contain the Cephalaspis Lyellii, 

 with another kindred species, and are therefore to be classed 

 with the Devonian rocks, though they must, under every cir- 

 cumstance, be viewed as the very base of that natural group. 

 It follows, therefore, that, if the great loAver conglomerates on 

 the flanks of the Grampians really underlie all those grey rocks 

 with pterygoti, cephalaspides, and Parha decipiens, they can no 

 longer be united as they have been with tlie Old Red or Devo- 

 nian, but must represent some portion of the Silurian system. 



" Lastly, I visited Dura Den, in Fifeshire, in the company of 



