32 F. J. AJ cock— The Athabaska Series. 



different type of topography from that which obtains in 

 the region today. The great thickness of the series 

 makes it necessary to postulate a down-warping basin or 

 basins with adjacent conntry of considerable relief from 

 which clastic material could be derived. The coarse 

 conglomerates and local breccias suggest torrential de- 

 posits carried but a short distance ; the greater part of 

 the series, however, was certainly transported distances 

 measured in miles. Fluviatile deposition on broad flood- 

 plains in intermontane basins seems to best represent the 

 conditions under which the series accumulated. 



The climate prevailing at the time the formation was 

 laid down may also be inferred. The prevailing red 

 color is due to the oxidation of iron and indicates condi- 

 tions of partial decay of granitic materials with regular 

 periods of exposure to the atmosphere. Such a condi- 

 tion is met with in a semi-arid climate in which periods 

 of rain alternate with periods of drought during which 

 a thorough oxidation of the iron is accomplished. 



The conditions of deposition may, therefore, be sum- 

 marized as follows : — broad subsiding basins between 

 mountains of considerable relief ; a semi-arid climate, in 

 which erosion and disintegration proceeded rapidly, load- 

 ing up the streams to their ultimate capacity ; torrential 

 floods in which the coarser materials were spread out 

 over the river flood-plains, followed by dry seasons in 

 which the sediments were exposed to the air, and their 

 iron content oxidized. The farther the materials were 

 transported, the more would the feldspar content be 

 sorted out, until finally the detritus would consist of 

 practically quartz grains only. Progressive subsidence 

 and rapid erosion would eventually give rise to a thick 

 series with all the features characteristic of the for- 

 mation. 



Late Pre-Cambrian time in Xorth America was appar- 

 ently marked by wide areal extent and conditions favor- 

 able for the accumulation of subaerial deposits. Rocks 

 similar to the Athabaska series are found on the east 

 arm of Great Slave lake, along Coronation gulf, and the 

 Coppermine river, on several of the Arctic islands, and in 

 the Labrador peninsula. The subaerial character of the 

 Keweenawan rocks of the Lake Superior region, and of 

 the Belt formation of Montana points also to the domin- 

 ance of continental sedimentation in late Pre-Cambrian 

 time. It is probable that in no subsequent period has 

 Xorth America had a greater areal extent. 



