294 Mr. MuRCHisoN on the Coal-field of Brora in Sutherlandshire, 



now submit to the Society the result of this comparison^ with some of the 

 illustrative fossils of each tracts accompanied by a map and sections. 



I shall commence with a description of that district which was the prime 

 object of my research^, and which is of the greatest importance in its mineral 

 contents. The Brora coal-field forms a part of those secondary deposits which 

 range along the S. E. coast of Sutherlandshire, occupying a narrow tract of 

 about 20 miles in lengthy and 3 in its greatest breadth. The strata connected 

 with it begin to rise from the sandy shore between Golspie and Dunrobin, 

 and extend to the mountain called the Ord of Caithness. For the purposes 

 of geological illustration, this tract may be said to be divided into three semi- 

 elliptical valleys, separated from each other by the successive advance to the 

 coast of the interior mountain range^ which in retiring, forms their W. and 

 N.W. boundary. These valleys diminish in size from S.W. to N.E,, and in 

 this distribution they stand thus : — 



1st. Brora. 



2nd. Loth. 



3rd. Navidale. 

 The first is bounded on the S.W. by the mountain of Ben-a-Bhraggle, 

 and on the West by the hills of Uppat, Loch Brora, and Clyne ; all of which 

 are composed of that red conglomerate which covers so large a portion of the 

 North-eastern Highlands. Here it is formed of fragments of all sizes derived 

 from the adjacent and more ancient rocks, passing into and alternating with a 

 fine-grained red sandstone, occasionally schistose, highly micaceous, and va- 

 riegated in colour. It is generally horizontal on the summits of the hills, or 

 deposited on their sides with a slight inclination to the East. It is quarried 

 as a building-stone at the base of the Craigton Hills, and the more fissile 

 schistose beds on the summit of Ben-a-Bhraggie have even been employed as 

 roofing-slates. On the N.E. of Loch Brora this red conglomerate gradually 

 passes inland in a N. W. direction, forming the cap of some of the higher 

 mountains. The low country of the coast is thenceforward bounded by 

 an unstratified granitic rock, which is first seen near the church of Clyne, 

 whence it sweeps down to the coast at Colyburn, there interrupting- the 

 stratified beds hereafter to be described, and thus inclosing the vale of 

 Brora. Again retiring from the coast it continues to mark the interior 

 boundary of the vales of Loth and Navidale, separating them by its ad- 

 vance at Porlgower, and terminating the secondary series at the Ord of 

 Caithness. This rock is throughout the greater part of its extent composed 

 of reddish felspar and gray quartz, which are more or less accompanied by a 

 decomposed green substance which may have been mica. It is always unstra- 



