164 



ice melted, they were left along with the associated material of 

 inorganic origin {Geol. Mag., 1874, p. 510). In the majority of 

 cases, as geologists have long remarked, these organisms from high 

 level drifts consist chiefly of the harder and less destructible 

 portions alone, and some of these themselves actually shew signs 

 of glacial striae. Mr. Clement Reid has called ray attention to the 

 fact that the assemblage of mollusca found in these deposits is such 

 as is never known to occur within the same bathymetrical zone. 

 This alone (without taking account of the anomalous admixture of 

 Celtic, Lusitanian, Boreal, and Arctic mollusca that characterises 

 such glacial shell-bearing gravels), is sufficient to put us on our 

 guard before accepting the evidence of these shells as proof of 

 submergence. On Mr. Reid's view, that large masses of frozen 

 mud, containing organisms, have been repeatedly transported con- 

 siderable distances in the ice and afterwards liberated as it melted, 

 there ought to be no difficulty in accounting for the deposition of 

 even delicate shells, unbroken, at any elevation reached by the ice, 

 and that is substantially the view I set forth in 1874. 



Fig. 8. 

 Diagram to illustrate the transportal of boulders in directions contrary to the 

 flow of the sole of the Ice- Sheet, (see pp. 131, 139, ante.) Ice carrying boulders 

 down hill and outwards from the centres of dispersal in a direction from A to B, is 

 met at B by ice currents setting in contrary direction (in most force at higher 

 levels). At B the opposing forces are balanced, and the direction of least 

 resistance is then mainly upward. Arrived at higher levels, the local current 

 becomes more and more affected by the influence of the currents setting inland, 

 and the ice then moves with its freight of mixed local and extraneous detritus 

 in the direction C, D. On the melting of the ice the sediment dispersed 

 throughout it is deposited on the rock surface below : the boulders both of 

 local and extraneous origin, between D and Z, for example, being sedimented 

 at Z. [It should be observed that in hardly any of the cases cited did the 

 direction of movement of the outflowing current lie diametrically opposite to 

 that of the current opposing it. The curve of transportal was probably hardly 

 ever entirely in the same plane. ] 



