GENERAL INFERENCES. 77 



the lowest Caithness beds to the upper sandstone of Dura Den, 

 in which several of the species reach the culminating point of 

 size and numbers, the whole intermediate suites of strata 

 contain more or less of the prevailing ichthyological fauna, and 

 are conveniently therefore grouped into one great geological 

 system. 



3. The clearly defined arrangements in the locality of Dura 

 Den and neighbouring district, may be studied with advantage 

 in working out the details of the system in other quarters. 

 The discussions, more especially in Ireland, respecting the car- 

 boniferous and yellow sandstone deposits, may be greatly 

 aided by an examination of the rocks in question. The term, 

 " yellow sandstone," I observe, is employed by Sir R. Griffith, 

 Jukes, and Kelly, indifferently to denote all the variety of 

 yellow-whitish rock embraced within the coal-measures. By 

 Scottish geologists it is restricted to the upper series of the 

 Old Red exclusively, and from shore to shore over Scotland, 

 with intermixtures of reddish marls, the prevailing colour is 

 yellow. The position in Ireland of a sandstone of the same 

 mineralogical features is identical with the rock of Dura Den, 

 and bed by bed, as described by Mr. Kelly, they occupy pre- 

 cisely the same identical relations to each other. " In the 

 carboniferous formation there are two black or grey shales, — 

 one below the main mass of the limestone, called carboniferous 

 slate, one above it called millstone grit, or, as it is sometimes 

 called, coal shale, because, in this country, it forms the base of 

 the coal-measures. There are two sandstones also, the Old 

 Red below, the top of which is yellow or white, and the sand- 

 stone of the coal measures above, which consists of several 

 bands, and which are also yellow or white, and separated by 

 black shales."'"" 



This is precisely the colour, nature, and relations of the 

 beds in Dura Den, and in all the adjoining range east and 

 west along the Knes of junction. Nor is the identity con- 



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