50 HISTORICAL GEOLOGY 



by deposition of the next series. Such repeated changes of relative 

 level between land and sea, as here recorded for Proterozoic time, 

 are among the most common and important phenomena of geo- 

 logic history. 



The Huronian rocks are principally quartzites, slates, schists, 

 limestone (usually dolomitic), and some conglomerates and beds 

 of iron ore, all of which are metamorphosed sediments. Locally 

 some of these beds have not been metamorphosed. Considerable 

 masses of igneous rock, some intrusive and some lava flows, also 

 are included among the Huronian rocks. The Lower and Middle 

 Huronian are usually much more metamorphosed and folded than 

 the Upper, the latter being at times scarcely at all deformed or 

 metamorphosed. Estimates show the aggregate (maximum) 

 thickness of the Huronian rocks to be no less than two or three 

 miles. 



The Keweenawan, or latest Proterozoic series, is characterized 

 by a great preponderance of lava flow which constitute the lower 

 portion of the series; are prominent in its middle portion; and are 

 practically absent from the upper portion. Some idea of the stu- 

 pendous and continuous volcanic activity of Keweenawan time 

 may be gained from the fact that lava sheets, mostly not over a 

 hundred feet thick each, accumulated to a depth of at least five or 

 six miles. Between some of the later lava sheets, thin beds of 

 sediment were deposited, while the upper part of the Keweenawan 

 consists altogether of sediments, chiefly conglomerates and sand- 

 stones. The sediments are estimated to have a thickness of about 

 three miles, so that the whole Keweenawan series must be some 

 eight or ten miles thick. 



Rocky Mountain Region. — Perhaps the largest known area of 

 Algonkian rocks in North America is that in the Rocky Mountains 

 of the northern United States and southern British Columbia. 

 These rocks generally rest upon eroded Archean and they are 

 overlain unconformably by Cambrian or still younger strata. The 

 rocks consist mostly of quartzites, sandstones, shales, and lime- 

 stones, more or less associated with igneous rocks. Their thick- 

 ness is at least two or three miles. Some of the strata (in Montana) 

 contain fossils. In central Montana at least the Algonkian strata 

 appear to have been upturned, folded, and somewhat eroded 

 before the deposition of the Cambrian. At present no satis- 

 factory subdivision of these rocks has been determined. 





