REVIEWS 117 



topography. Fossils, stratification, terraces, and silted divides occur 

 at all altitudes below 500 feet, but never above, and although the 

 greater abundance of the marl-loess on the east side of the valley is 

 suggestive of eolian action, the character and range of the features 

 mentioned afford a preponderance of evidence in favor of an aqueous 

 origin of the deposit up to an altitude of 500 feet, or 120 feet above 

 the river. The upland loess is regarded as an eolian derivative of the 

 marl-loess. 



The Ha7igi7ig Valleys of Qeorgetow?i, Colo. By W. O. Crosby. 



The paper describes chiefly the break of several hundred feet 

 between the floor of the valley of Clear Creek and that of one of its 

 principal tributaries, Leavenworth Creek, and explains it as due, not to 

 fluvial or glacial erosion, but to faulting, of which abundant inde- 

 pendent evidence is afforded by mining developments. Other and 

 similar features in the vicinity are correlated with this, and it is shown 

 that the part of the main valley occupied by Georgetown is a depressed 

 fault block or graben, and that the valley is due, in part, to displace- 

 ment, and not solely to erosion, suggesting comparison with Yosemite. 

 The idea is also advanced that the elevation of this part of the Colo- 

 rado Range has been recently, and may be still, in progress, and that, 

 while in the past the movement has been chiefly massive, developing 

 the great fault scarp overlooking the plains, it has, in later time, 

 affected the axis more than the margin of the great orographic block, 

 leading to a marked tilting of the Cretaceous peneplain, and, in part 

 at least, it is very locally differential, and, in the Georgetown instance, 

 in a way to accentuate the topography. 



Glacial Features of Lower Michigan. By Frank Leverett. 



This paper presents results of an investigation of the Pleistocene 

 deposits and features of Michigan carried on for the past three years 

 under the supervision of Professor T. C. Chamberlin, chief of the gla- 

 cial division of the United States Geological Survey. 



Lower Michigan lies within the limits of the latest or Wisconsin 

 drift sheet, but inequalities of earlier drift sheets may have given rise 

 to some of the topographic features and possibly to the strong fea- 

 tures, such as basins and the high bordering rims. The high country 

 northwest of the Saginaw basin has 350 to 500 feet or more of drift, 

 while the basin itself has an average of scarcely 100 feet. It seems 



