168 PROCEEDINGS OP THE GEOLOGICAL SOCIETr. [Jan. 11, 



to a later period, and are superimposed upon the irregular undulat- 

 ing stirface of this old boulder-earth. 



It is evident, however, that preexisting organisms, whose 

 remains lay on the surface before the advance of the glacier, might 

 be mixed up with the other superficial debris and carried along by 

 it, and thus broken shells of the Crag-period, and remains of the 

 Mammoth, may have come to be imbedded in the Boulder-clay. In 

 the Boulder-clay of Norfolk broken bits of Crag shells are com- 

 mon, and in that of Yorkshire and of Scotland the tusks of the 

 Elephant have occasionally been got. 



If the whole country was buried under a thick covering of snow, 

 it is clear that no proper moraines would be formed. Moraines are 

 deposited along the outer edge of the ice, and consist for the most 

 part of the debris hurled down upon its surface from the rocky slopes 

 and precipices overhanging the glacier. This mass of stony rub- 

 bish lying on the top is not scratched and worn like that which lies 

 beneath the ice; for it floats, as it were, on the surface, and is 

 deposited quietly at the end (and sometimes along the sides) of the 

 glacier. It is the stuff caught between the ice and its rocky bed 

 that is rubbed and worn ; the debris on the surface is not scratched. 

 Now if the ice covered the whole land, so that no rocky cliffs pro- 

 truded through it to send down their debris upon its surface, it is clear 

 that there would be an absence of all this superficial stony rubbish 

 which goes to form the moraine of a Swiss glacier of the present day. 



In Aberdeenshire this old boulder-mud is of a dull greyish tint, 

 such as might be derived from the trituration of the metamorphic 

 schists and crystalline rocks. It may be traced from the shore at 

 the Bay of Nigg all up the valley of the Dee for sixty miles inland, 

 and from the sea to the height of 1500 feet, everywhere of very 

 much the same general hue and character; the stones in it are 

 often well rounded, some of the granite ones being nearly as round 

 as cannon-balls. Prom Stonehaven to the banks of the Leven in 

 Fife the Boulder-clay is reddish, ovdng to the broad zone of red 

 sandstone which the ice had to pass over. In the basin of the 

 Perth it is dull grey in the upper part of the vaUey (near the Loch 

 of Monteith, for example), where the debris consists of stuff from 

 the old crystalline rocks ; near Stirling it is reddish brown, from 

 the influence of the red sandstone ; at Palkirk it is a deep brown, 

 becoming blackish towards Edinburgh, owing to the gradually in- 

 creasing effect of the debris from the coal-strata. 



The ice that overspread Perthshire, as it moved south-east, carried 

 along the boulders of Grampian mica-schist, and mixed them up 

 with the red sandstone of the Lowlands, next with the trap of the 

 Ochil Hills, and finally with the fragments of the coal-beds, until 

 on the shores of the Pirth of Perth it has left a medley of aU 

 the different kinds. 



The granite-boulders from the Ben Muick Dhui mountains have 

 been thrown in profusion north-westward into the valley of the 

 Spey — even crossing that valley, and lying in thick beds high up 

 on the slopes of the hills to the north of Aviemore ; they have also 



