176 PEYSICAL GEOGRAPHY. 



force northwards would produce the basin of the Mac- 

 kenzie, commencing on the 53rd parallel, but in a district 

 narrowed and disturbed by the approximation of the in- 

 termediate primitive rocks to the Rocky Mountains. 



Supposing the continent to have retained its present 

 form since the era of these excavations, it seems scarcely 

 possible to reconcile the existence of extensive glacial action 

 with any modification of climate ; yet the smoothened sur- 

 faces, streaks, and furrows referred to that action, whether 

 in the form of glaciers or of drift ice, are of no rare 

 occurrence, wherever durable rocks show themselves, be- 

 tween the St. Lawrence and the Arctic Sea. 



In connection with the excavations of the North Ame- 

 rican continent, the fact may be mentioned of the great 

 indentations of the coast line, including Hudson's Bay, 

 the Gulfs of St. Lawrence and Mexico, and the Caribbean 

 Sea being on the east side ; while breaks of the west shore, 

 to the south of the peninsula of Alaska, are comparatively 

 small, and both coasts of South America are nearly entire. 



The geologists of the New York Survey consider that the 

 present continent of North America was constructed from 

 the debris of land lying more to the eastward. Mr. Hall, 

 speaking of the strata exposed between the Hudson and 

 the Mississippi, states that they contain organisms which 

 must have lived in the bed of the ocean, and that the chief 

 source of the sedimentary deposits lay to the east and 

 south-east. To the westward, the sedimentary rocks 

 are of a finer grain, and at the same time diminish in 

 quantity, while the carbonate of lime increases, indicating, 

 in conjunction with the contained fossils, the bottom of an 

 ocean of greater depth and more quiet condition. The 

 cretaceous and tertiary deposits of the western prairies 

 show, according to the geologist just quoted, that the 

 eastern part of the continent was first elevated, and that 



