1848.] NICOL ON RECENT FORMATIONS NEAR EDINBURGH. 21 



tons in weight, and generally derived from trap, sandstone, or lime- 

 stone rocks, like those composing the coal-field on which it rests. 



Fig. 2. 



Some of these boulders however consist of granite, mica-slate or 

 other primary strata, and must consequently have been carried a 

 greater distance, as none of these rocks are found nearer than from 

 forty-five to fifty miles, and granite in any quantity only at seventy 

 miles' distance. These boulders are generally rounded and water- 

 worn, but some on the contrary are angular. They are found in 

 every part of the mass of blue clay, but, as it seemed to me, in more 

 abundance in certain portions, and apparently arranged in horizontal 

 lines. 



These facts appear to prove that the deposition of this boulder 

 clay or till was gradual, — the effect of long-continued and variable 

 agents ; and not of a sudden rush of water, or debacle, as has been 

 imagined. The whole phsenomena seem more consistent with the 

 supposition that the clay was formed by the continuous action of the 

 sea on the various strata of the subjacent coal-field, than with any 

 other theory. The blue clay forming the great bulk of the till may 

 be regarded as merely the decomposed shales of the coal formation, 

 and the sands as comminuted sandstones : even the relative position 

 of the deposits, with the blue clay below, and a browner and more sandy 

 clay resting upon it, as seen in the sections, favours this opinion. The 

 soft shales when exposed to the action of the waves would be wasted 

 away before the harder sandstones and trap rocks, and the deposit 

 formed from their destruction would consequently occupy a lower 

 position. The boulders may have been brought to the place where 

 we now find them by ice, or entangled in the roots of floating trees, 

 or in any other mode now in action for the transport of rock masses. 

 Though mixed up irregularly with the mass of clay, it is by no means 

 necessary that they should have been always transported along with it 

 or by the same agent. Were a number of boulders at the present 

 day dropped on a mass of soft semifluid clay at the bottom of the sea, 

 they would not remain on the surface, but sink in it to various depths, 

 and thus appear to have been deposited by the same agents, when in 

 reality they were deposited by wholly different causes. Neither does 

 the apparent want of stratification in the clay prove it to have been 



