THE GEOLOGY OF THE ENGLISH CHANNEL. 159 



M. 51a. S. 15 W. Edd., 30 8 miles. 



Wliile this paper is in the press the writer has examined a rock 

 from M. 51, which he had previously overlooked. This is a compact, 

 dark brown limestone, with large shell fragments, now in calcite. The 

 stone is angular. It is but little softer than M. 80e, and when sectioned 

 shows dendritic markings similar to those in the lighter portion of 

 that specimen. Undoubtedly liassic in type it in some sort forms a 

 stepping stone from the more frequent forms to M. BOe, and the latter 

 may now with fair certainty bo identified as liassic. In mapping pur- 

 poses this has been assumed. 



GEOLOGY. 



In the preceding section the various rocks have been assigned to 

 their respective formations and their peculiarities noted. In Mr. 

 Crawshay's paper their mode of occurrence, independently of their 

 lithology, has been fully described. It remains to construct from the 

 evidence thus called some coherent scheme of geology for this portion 

 of the Channel. 



With this end in view it is especially necessary to consider the 

 probable date of these stony accumulations as such, and to find some 

 reasonable explanation of their presence. Following which we must 

 be assured that to some considerable extent the deposits are of local 

 origin before we can proceed to any mapping of the various forma- 

 tions under the waters of the English Channel. 



The one outstanding feature, as Mr. Crawshay has pointed out, is 

 the general increase in average weight and size of the stones due 

 south (magnetic), i.e. straight outwards into the Channel from the 

 Eddystone. But although, as regards the dredged material, this fact 

 is clear and important, against it or with it must be set the occurrence 

 of large boulders on the Salcombe and Eddystone fishing grounds. 



That the grade of the bottom deposit should grow coarser as the 

 distance from land increases is against all probability and all experience, 

 if the deposit is of recent formation under existing conditions. It should 

 be expected that the detritus which enters the sea by the mouths of the 

 rivers, derived from the denudation of the land, would so sort itself that 

 the heavier and coarser particles deposit in the nearer and shallower 

 waters, the lighter and finer particles coming to rest in the deeps ; 

 and, added to the river-borne detritus, the products of coast erosion 

 and broken shells from the littoral zone should similarly distribute 

 over the sea-bed with reference to the weight and size of their grains. 

 No matter how small the rivers, how slight their supply of sands and 

 clays, and without reference either to the rate of erosion and supply 

 from the sea-clifis, in time and in the absence of strong localised 



