OF NORTH AMERICA. 23 



"The originally dry plains of the eastern part of the state are 

 almost unbroken up to their very abrupt meeting with the foothills 

 of the Front Range, at least from Colorado Springs to the northern 

 boundary of the state. Through the plains run occasional streams, 

 along which were originally some cottonwoods, willows, etc., but, 

 generally speaking, except close to the borders of the streams, the 

 country is semi-arid, with occasional shallow lagoons or lakes, often 

 of considerable area but seldom of any depth, many of them being 

 dry through a large part of the year, but filling up during the spring 

 rains and run-off from the snows. These lakes are believed to be 

 largely wind-formed, though that is not at all certain in many cases. 

 At any rate, they are not of glacial origin, as the glaciers in - this 

 region did not extend below the 7000-8000 foot line, while the plains 

 meet the foothills at from 5000 to 6000 feet. 



"Wherever the water from the streams can be economically 

 carried in open ditches, by going upstream far enough to get sufficient 

 head, a great change has been wrought in the character of the country 

 in the last forty years, much of it within the last twenty. Many of 

 the lakes have been enlarged and deepened by the building up of their 

 dams, other reservoirs have been formed, dotting the plains everywhere 

 within convenient reach of the streams, and ditches extending in all 

 directions. Seepage water from ditches and reservoirs form many 

 smaller pools, which are apt to be dry except during the irrigation 

 season, just as the numerous pools formerly formed by overflow water 

 along the streams during the spring run-off dry up later in the season. 

 Many of the lakes are choked with vegetation usually found in such 

 situations, while others are quite free therefrom. The bottom is apt 

 to be of fine mud. In the mountains above 8000 feet there are nu- 

 merous lakes, the origin of some of the lower ones being in doubt, 

 but all the higher lakes being of glacial origin. The lower lakes, the 

 only ones in which I have found Lymnseas, are usually very shallow 

 lagoons, choked up by vegetation, including sedges, etc., and nearly 

 or quite drying up in late summer." 



Dr. Henry A. Gleason has recently 1 made an interesting ecological 

 survey of Isle Royale, Michigan, and his remarks on the ecological 

 relations of the Lymnseas of this area are of great value. He says: 



"Tonkin Bay is a small inlet about a half a kilometer long, opening 

 to the east upon the lake, and with steep, approximately parallel sides. 

 It is narrowed half way up by two beaches lying perpendicular to its 

 length. By this the wave action on the upper part is reduced, but 



Rep. Ecol. Surv. Mich., 1908, pp. 60-62. 



