THE TEEEACES OF BEITISH COLUMBIA. HI 



cient detail to fix precisely the boundaries of all these ancient lake systems. 

 There can be no question, however, in regard to the main looint, namely, 

 that the Great Basui was once covered in large part by water, which formed 

 lakes of various dimensions but of complicated ontline, some of which were 

 equal in area to the Great Lakes of the eastern side of the continent. The 

 present aspect of the region in question is one of extreme aridity. The 

 existing lakes do not occupy one tenth part of the area formerly covered by 

 water. They are not bodies of fresh water ; but are saline and alkaline, as 

 must necessarily be the case in a region where evaporation is in excess of 

 precipitation, and where there is consequently no drainage of superfluous 

 water to the sea. No one can doubt that we have here evidences of an im- 

 portant climatic change occurring in later geological times, and in regard to 

 which several interesting questions suggest themselves, having reference to 

 the epoch of this change, its duration, and its relation to other manifestations 

 of a similar character, or at least leading to similar inferences in their appli- 

 cation to the topics up for discussion in the present volume. These ques- 

 tions, however, it will be best to defer until more facts have been brought 

 forwai'd from other portions of the Cordilleras, outside of the limits of the 

 Great Basin. 



Next to the Great Basin proper, British Columbia may perhaps be re- 

 garded as offering the most remarkable indications of a change in climatic 

 conditions during the later geological periods. As already mentioned, the 

 streams throughout that part of the country embraced between the Rocky 

 Mountains and the Coast Ranges, north of the United States boundary line, 

 are bordered with numerous terraces, which remain in perfect preservation, 

 and prove beyond the possibility of doubt that the amount of water carried 

 off to the sea was vastly larger than it now is. The streams at the present 

 time are often almost a continuous succession of lake-like expansions ; but 

 this peculiar phase of fluviatile conditions as now exhibiting itself in British 

 Columbia is but a foint shadow of what it formerly was. We have at present 

 but little idea of the depth of the lakes in this region : there cannot be much 

 doubt, however, that some of them will be found to occupy depressions sunk 

 to considerable depth below the level of the surfaces of the rivers them- 

 selves. The valleys are manifestly orographic in character; and, that being 

 the case, portions of these are likely to be very deep, where so situated as 

 not to have become filled up with debris. Briti.sh Columbia exhibits, then, 

 at the pi'esent time, a transition stage between almost complete inumlation. 



