762 



GEOLOGY. 



Block in Pembrokeshire. 



The facts ascertained respecting the dispersion of drift and of erratic blocks, comprise 

 the following phenomena : 



1. Evidence appears of the drift, using the term comprehensively, having been carried 

 outward from the summits and axes of particular mountains, and spread over the plains 

 and valleys in their neighbourhood. The best example of this occurs in the Alps, where 

 boulders have been usually carried down the valleys, and exist in the greatest abundance 

 opposite the lower openings of those valleys. Similar instances have been pointed out 



among the mountains of Scotland 

 and thoso of the north of England. 

 The plain of Metidja, on the south 

 of Algiers, is described by Rozet, as 

 covered in its northern parts by 

 boulders derived from a long chain 

 of hills running along its northern 

 border, while its southern region is 

 strewed with blocks from the Atlas 

 chain which stretches along its south- 

 ern limits. 



2. Viewed upon a larger scale, 

 the drift affords evidence of having 

 been dispersed in a general southerly 

 direction over all the northern he- 

 misphere, often to a great distance, 



variously modified, however, by local obstructions. Dr. Buckland mentions pebbles and 

 blocks of granite and seinite, of a very peculiar character, .drifted from the Criffle 

 mountain in Galloway, across the Solway Firth, and scattered over the plain of Carlisle ; 

 while pebbles, and large blocks of another kind of granite, have been drifted in still 

 greater numbers from Ravenglass, on the west of Cumberland, over the plains of 

 Lancashire, Cheshire, and Staffordshire, upon which they lie in masses of several tons 

 weight. Professor Sedgwick states that the blocks of Shap granite, near Penrith, which 

 cannot be confounded with other rocks in the north of England, are not only drifted 

 over the hills near Appleby, but have been scattered over the plains of the new red 

 sandstone, rolled over the great central chain of England, into the plains of Yorkshire, 

 imbedded in the transported detritus of the Tees, and even carried to the eastern coast, 

 a direction from the parent bed south by east. Another current of drift, found in the 

 east of England, consisting of varieties of primitive and transition rocks which do not 

 occur in this country, the origin of which must be referred to Norway, has pursued a 

 course south by west. On the Continent the evidence of a southerly direction is very 

 apparent, the blocks and pebbles that are strewed over the plains of North Germany, 

 Poland, and Russia, having their parent rocks in Sweden, Lapland, and Finland. Across 

 the^ Atlantic the boulders spread over the southern part of Nova Scotia have been 

 derived from the ledges in the northern part of the province ; and throughout Maine 

 and Massachusetts the direction taken by the drift, as shown by a multitude of examples, 

 is from north to south, varying a few points towards south-east or south-west. In many 

 parts of the states south of the western lakes, the surface is bestrewed with an immense 

 number of fragments of primitive rocks, significantly called " lost rocks," which may be 

 traced to the north side of the lakes in Upper Canada. The great valley of the Mis- 

 souri and Mississippi, from the Yellow- stone river almost to the Gulf of Mexico, is 

 represented by Mr. Catlin as covered with vast quantities of blocks of primary rocks, 

 which have been drifted thither from the north-west. He describes five remarkable 



