NATURAL HISTORY SOCIETY OF GLASGOW. 339 



stream of shorter length, but ampler volume, than that of the 

 present Clyde. The amount and character of that drainage, and 

 the quantity of sediment or mud it brought with it, can be only 

 roughly guessed at from the condition of the land. The ice sheet 

 had lifted from the lowlands, but still lay in patches upon the 

 higher grounds, and from it local glaciers descended by the minor 

 valleys to the sea. The higher parts of the Kilmalcolm, the 

 Eaglesham, the Kilpatrick, and the Campsie hills were probably 

 still shrouded in snow and ice, and down their glens the drainage 

 flowed, first as ice, but gradually changing into water. The upper 

 parts of Clydesdale, surrounded by even higher hill ranges than 

 the lower, were also in like manner doubtless covered with ice, 

 and yielded much glacial water. All glacier streams are full of 

 mud — the finer waste of the land. This mud would begin to sink 

 as soon as it came into still water, precipitation being quickened 

 where the saltness was sufficient to make the water brackish, as 

 shown by the experiments of Mr David Robertson. Thus were 

 the great beds of muddy clay formed. The occasional boulders, 

 and numerous smaller stones often found with them, would be 

 brought by ice from the shore — by icebergs and ice-floes, 

 and dropped as they melted or foundered. At first, imme- 

 diately after the submergence, the deposition of the mud was 

 probably rapid, and the lower portion of the muddy clay was 

 formed in much less time than the upper, perhaps even before the 

 marine fauna of the great sea outside the land had time to enter 

 in and possess this little inland sea. But however that may be, 

 the whole time occupied in the deposition of the muddy clay was 

 doubtless a long one, and the conditions, though varying at times, 

 were in the main similar throughout. In the very thickest of the 

 shell-beds boulders occur, and striated stones are frequent; showing 

 that much ice was abroad in the sea in which the Arctic shells 

 lived. The grouping of the shells, also, in some of the beds — 

 and notably in that of Houston — indicates also, I think, the action 

 of ice as a carrying agent. Many of the shells belong to littoral 

 species, and are found living only between tide-marks, on stony 

 or rocky shores ; yet here we have them deposited among mud, 

 alongside of shells which inhabit deep water. How does this 

 happen 1 I suppose it to be by the shore shells becoming frozen 

 into shore ice, and lifted by it and carried seawards. From this, 

 or other causes, so large a proportion of the shells found at 



