184 THE STORY OF THE EARTH. 



rots upon a glaciated and grooved surface of 

 rock; (ii.) The Lower and Upper Leda-clay with 

 marine shells and drift plants ; and (iii.) an 

 Upper Boulder Clay with the shell Saxicava % and 



. el. The Lower Boulder Clay forms the 

 basins of the great Canadian lakes. The boul- 

 ders are mostly of Laurentian gneiss. Their 

 striation is attributed to the grating of pebbles 

 included in shore-ire upon the rocky lloor be- 

 neath, when moved by the tide. 



In Britain the glacial deposits are spread 

 irregularly. They consist of Upper and Lower 

 Loulder Clays on the east coast, divided by 

 Middle Glacial Sands with marine shells. 



The granite of CrilTel in Kirkcudbrightshire 

 is found in the boulder clay over Lancashire and 

 North Wales. Boulders of Volcanic rocks from 

 Cumberland are scattered over Cheshire. Dis- 

 tinct streams of glacial drift extended down both 

 the east and west sides of Britain. The boulders 

 of Westmoreland Shap granite found over the 



plain of York and between Whitby and Scar- 

 borough on the coast, prove that boulders were 

 1 distributed eastward fr<>m local centres, not- 

 withstanding the Scandinavian source of many 

 rocks in the Boulder Clay on the Norfolk Coast 

 The Boulder Clay found near Lon- 



Finchley, and at Hornchurch in Essex, is 



full Of travelled ice-grooved locks, with fossils 

 ondary strata of Yorkshire a:ul Lin- 

 colnshire. Glacier ice transported the rock mat- 

 but probably shore-ice and icebergs \ 

 tly concerned 111 depositing it, so as to fill up 



the old valleys and leave the clay on the suit 



<>f the country. The denudation s'mre the ghe 



id has been ve I , and the giaeial beds 



