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GEOLOGY. 205 



tlicin which arc buried in the soil, and, in this way, protcclcd ; 

 their angles and corners show but little erosion. On the 

 other hand, wherever there is a deposit of pebbles, its origin 

 may be easily traced to local causes tliat have acted long 

 after the arrival of the erratic deposit now under considera- 

 tion. 



" It is difficult to determine the direction whence the ma- 

 terials of the erratic deposit came. The presumption is, 

 judging from the nature of the erratic blocks — the analogues 

 of which are found in higher latitudes — that they were 

 brought from the north to the south. 



*' On the borders of the great lakes, on the flanks of valleys, 

 and where traces of recent floods are apparent, the erratic 

 blocks are in great abundance. Their size varies from a few 

 inches to a few cubic feet ; yet this seems to bear no relation 

 to the distance whence they are supposed to have come. On 

 the contrary, it appears that the largest are often found in the 

 highest spots, and at a greater distance from tlieir origin. I 

 did not find them more abundant on the northern slopes of 

 hills than on the southern. Their oryctognostic distinctions 

 are — granitic sienite, resembling the Egyptian red granite ; 

 a true sienite, with white feldspar ; a gi'anite, with a large 

 proportion of feldspar ; gneiss, amj/liibolite, red jasper, quartz 

 pebbles, and a great variety of agates and cornelians. These 

 last are carried off" by the streams, and scattered in great 

 numbers over the shores of the Missisippi and Missouri ; but 

 they have no value, and are collected by travellers merely as 

 reminiscences. The sand and gravel are composed of the 

 small fragments of all these rock^ ; the sand, though varying 

 according to places, being principally siliceous. It is this 

 sand which constitutes the predominant ingredient in the soils 

 of the whole region embraced in the map, modified according 



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