Correspondence — Dr. C. Ricketts. 141 



to 600 or 700 feet in North Lancashire, and supposed by some to be 

 shown by more doubtful deposits at greater heights. 



This comparatively mild period gave place to a sea crowded with 

 floating ice, wherein the Upper Boulder-clay was deposited on the 

 Gravels, and perhaps on the highest islands glaciers were forming 

 and sending off little icebergs into the channels of an archipelago. 

 But the ice of this cold period never attained sufficient strength to 

 plough out of the' arms of the sea the sand and gravel of the pre- 

 ceding mild period. It could not therefore have been the agent 

 which swept the remains of the older mammals off the face of the 

 district, but probably belonged to the latest cold period in Britain, 

 and I can only say that that of the Ice-sheet is the earliest of which 

 we have any traces left in Lancashire and West Yorkshire. 



Universities' Club, Jermyn St., S.W. R- H. TiDDEMAN. 



ON SUBSIDENCE AS THE EFFECT OP ACCUMULATION. 



Sir, — Will you permit me to make a few remarks upon the critique 

 on " Valleys, Deltas, Bays, and Estuaries," ^ in the last Number of 

 the Geological Magazine ? 



The reviewer charges me with attributing to such small accumu- 

 lations, as those of "a delta, a shingle-beach, or the ice and droppings 

 of a glacier," the power of weighing down gradually the crust of 

 the earth. Such an opinion would, indeed, be " pushing a theory 

 too far," even to absurdity. 



The accumulation in a delta represents a comparatively infini- 

 tesimal portion of the debris, derived from the disintegration of the 

 material, formerly occupying the space that constitutes the area of 

 a valley, and which has been removed during the process of its 

 formation, having been carried down by the river and deposited near 

 its mouth and in the neiglihouring sea. 



It is considered that during the Glacial Period, there were not 

 simply Glaciers in Britain, but that the country was enveloped in a 

 mantle of snow and ice, similar to what now exists in Greenland ; 

 and it has been estimated that in some parts it must have attained a 

 thickness of at least 2000 feet. Supposing the weight of this mass 

 was the same as that of a similar depth of water, it would indicate 

 an increase of pressure on the surface of the land amounting to 

 937 lbs., or about eigJit liiindredw eight and a half to the square inch. 



Dr. Piobert Brown (Quart. Journ. Geol. Soc, xxvi., p. 681), states 

 that he can find no appreciable difference between the deposit of the 

 mud, with which in Greenland the sub-glacial streams are loaded, 

 and that of the clay of the Boulder-clay, and it was to such a source 

 I attributed its formation. Though its thickness in this neighbour- 

 hood is in places considerable, even after much denudation, as its 

 deposition occurred close to a land-margin, it will jDrobably at all 

 times have been moderate compared with that which lies beneath 

 the waters of the Bay of Liverpool. "The droppings of glaciers," 

 i.e. the scratched and other boulders and pebbles contained in the 



' An abstract of tliia Essay was given in the Geological Maca/ine, Vol. IX, p. 119* 



