100 ANNALS NEW YORK ACADEMY OF SCIENCES 



deposit consists of the extraordinary distribution of large and small 

 bowlders within a rock which is otherwise fine enough to be classed as a 

 shale or very fine sandstone. The unusual occurrence of the scattered 

 bowlders of different sizes calls for a special explanation. Dr. Fred J. 

 Pack^^ of the University of Utah has suggested a glacial origin for the 

 bowlders that occur in a similar fine-grained black rock on Stansbury 

 Island in Great Salt Lake. At that place, they show facetting suggesting 

 ice erosion, though glacial strige have not been seen. In the Cottonwood 

 deposit, the bowlders are of greatly varying sizes, from small pebbles to 

 blocks weighing several tons. The smaller ones show water action in that 

 they are much rounded, but the extremely large ones are distinctly angu- 

 lar. It is difficult to imagine how such masses came to be imbedded in a 

 rock which is otherwise so uniformly fine grained, unless we appeal to an 

 agent like ice which has the power to carry them long distances and de- 

 posit them without changing their form to any marked extent. Ice- 

 rafting might be suggested as a possible means for their irregular distri- 

 bution. Facetting and more or less complete rounding of the smaller 

 pebbles would also be conceivable on such an hypothesis. In fact, the 

 heterogeneous nature of the deposit is one of the strong factors in support 

 of the supposed glacial origin (see Plate II). 



In the March number of the American Journal of Science for 1907, 

 Coleman,^^ after a brief review of the reported occurrence of Paleozoic 

 ice ages in many parts of the world, suggests the probable existence of a 

 Lower Huronian ice age : 



"For several years, it has seemed to me very probable that there was a still 

 more ancient ice age, at the beginning of the Lower Huronian in the Archean 

 as defined in Canada or the Archeozoic or lowest Algonkian as defined by 

 various American geologists. The so-called Huronian 'slate conglomerate' of 

 Ontario has attracted attention ever since Logan and Murray mapped and 

 described it in the typical region north of Lake Huron nearly fifty years ago, 

 Grood descriptions of it are given by Logan in the 1863 report of the Canadian 

 Geological Survey ; where he refers to the different kinds of rock inclosed as 

 pebbles or boulders, granite, felsite, certain green-stones and jaspers, for ex- 

 ample ; and describes the matrix as sometimes slaty, sometimes more quartzitic 

 or like diorite or green-stone. At present the matrix would be called gray- 

 wacke or slate though sometimes it is schistose or looks like an eruptive rock. 



"The pebbles are in many cases subangular or sharply angular and are 

 found miles away from any known source ; and as they may be of any size up 

 to blocks weighing tons, and are frequently very sparcely scattered through 

 an unstratified matrix, a stone or two in several yards, one cannot help suspect- 

 ing that the transporting agency was ice rather than water." 



^1 Personal communication. 



"A. P. CoLEMAX : "x\ Lower Huronian Ice Age," Am. Jour. Rci.. Vol. 28, p. 187. 

 1907. 



