130 



FOSSILS OF AECTIC SPECIES. 



[Ch. XI 



Tariable deptli ; for the heavier erratics require icebergs of a larger size 

 to buoy them np ; and even when there are no stones frozen in, more 

 than seven-eighths, and often nine-tenths, of a mass of drift-ice is under 

 water. The greater, therefore, the vohime of the iceberg, the sooner 

 would it impinge on some shallower part of the sea ; while the smaller 

 and lighter floes, laden with finer mud and gravel, may pass freely over 

 the same banks, and be carried to much greater distances. In those 

 places, also, where in the course of centuries blocks have been carried 

 southwards by coast-ice, having been often stranded and again set afloat 

 in the direction of a prevailing current, the blocks will diminish in size 

 the farther they travel from their point of departure for two reasons: first, 

 because they will be repeatedly exposed to wear and tear by the action of 

 the waves ; secondly, because the largest blocks are seldom without di- 

 visional planes or "joints," which cause them to spht when weathered. 

 Hence as often as they start on a fresh voyage, becoming buoyant by 

 coast-ice which has frozen on to them, one portion of the mass is detached 

 from the rest. A recent examination (in 1852) of several trains of huge 

 erratics in lat. 42° 50' N. in the United States, in Berkshire, on the west- 

 ern confines of Massachusetts, has convinced me that this cause has been 

 very influential both in reducing the size of erratics, and in restoring an- 

 gularity to blocks which would otherwise be rounded in proportion to 

 their distance from their original starting point. 



The " northern drift" of the most southern latitudes is usually of the 

 highest antiquity. In Scotland it rests immediately on the older rocks, 

 and is covered by stratified sand and clay, usually devoid of fossils, but 

 in which, at certain points near the east and west coast, as, for example, 

 in the estuaries of the Tay and Clyde, marine shells have been discovered. 

 The same shells have also been met with in the north, at Wick in Caith- 

 ness, and on the shores of the Moray Frith. The principal deposit on 

 the Clyde occurs at the height of about 10 feet, but a few shells have 



Fig. 110. 

 Astarte borealis. 



Fig. 111. 

 Leda oblong a. 



Fig. 112. Fig. 113. Fig. 114. Fig. 115. 



Saxicava rugosa. Peeten islandictis. Mitica clausa. TropJion dathratum. 



Northern shells common in the drift of the Clyde, in Scotland. 



been traced in it as high as 554 feet above the sea. Although a propor 

 tfion of between 85 or 90 in 100 of the imbedded shells are of recent 

 species, the remainder are unknown ; and even many which are recent 



