SHELL BEDS. 47 



22. The arctic or sub-arctic climate which prevailed in the 

 circumstances we are considering would cover the hill-tops 

 with perpetual snow; glaciers would fill the heads of the 

 valleys, then greatly reduced in length by the submergence 

 of the lower lands, and the marine fauna of the period would 

 flourish under and outside the rim of ice which girt the 

 island ; while bergs and floes would float away into the open 

 water, bearing up buoyant for a time, and then throwing 

 down their load of blocks to encumber the southern plateau, 

 as already pointed out (Art. 20). The disturbances attendant 

 on the re-elevation of the land could not fail to produce a 

 certain re-arrangement of the surface beds, and even occa- 

 sional intermixture, and the heaping up of detritus in peculiar 

 forms, such as those already noticed in the case of Glen lorsa. 

 The boulder-clay, by its position, would in many cases be 

 protected from such action, while in some situations more 

 exposed its upper surface would be subjected to great erosion 

 or removal, and hence, no doubt in part, the varying depth 

 and undulating surface of this deposit. Its original forma- 

 tion was most probably contemporaneous with the incipient 

 depression of the land, and due to the joint action of ice and 

 water ; while its internal structure, huge boulders, unstrati- 

 fied character, and the almost total absence of shells, indicate 

 a period of violent disturbance, a hurried pell-mell admixture 

 and deposit of earth and rocky materials. The angular form 

 of many of the blocks, the perfect and very general striation 

 of the angular and rounded alike, so that in many places 

 non-striation is the exception, cleai-ly shew that, by whatever 

 agency this singular deposit was formed, and the striation 

 effected, there cannot have been afterwards a lengthened 

 transport by rivers, as this is known in the case of the 

 Alpine streams to produce a speedy obliteration of such 

 markings. 



Further details regarding this curious deposit will be given 

 in one of our Excursions, and the subject of the shell beds 

 and the order of superposition of the clays treated at greater 



