362 PRE-CAMBRIAN PERIODS 



the present, at least, it is better to form a separate grand division 

 for them. 



The Algonkian rocks, which are widely distributed in North 

 America, form an immensely thick mass of strata and of metamor- 

 phic rocks which are believed to represent the strata. These 

 metamorphic rocks have hitherto been generally referred to an 

 upper division of the Archaean, called the Huronian, but, so far 

 as can be learned, they occupy the same stratigraphical position 

 as certain little changed sediments, between the fundamental com- 

 plex below and the Cambrian above. At the base of the magnifi- 

 cent section exposed in the Grand Canon of the Colorado is a 

 very thick mass of strata, separated by great unconformities from 

 the Archaean gneiss below and from the overlying Upper Cam- 

 brian. This mass is again subdivided by minor unconformities 

 into three series. The lower series, at least 1000 feet thick and 

 perhaps more, is made up of stratified quartzites and semi-crystal- 

 line schists, cut by intrusive granite. Above this come nearly 

 7000 feet of sandstones, with included lava sheets, and at the top 

 more than 5000 feet of shales and limestones, in which a few fos- 

 sils have been found. The two upper series are not at all meta- 

 morphic. All these strata are steeply inclined, and upon their 

 edges rests the Upper Cambrian. 



A very similar succession of rocks of vast thickness is found 

 in the Lake Superior region, intervening between the Archaean 

 complex and the Upper Cambrian, from both of which they are 

 separated by great unconformities. As in the Grand Canon 

 section, these rocks are divisible into three series by minor un- 

 conformities. The lowest, with a maximum thickness probably 

 exceeding 5000 feet, is much crumpled, metamorphosed, and 

 semi-crystalline. It comprises limestones, quartzites, mica schists, 

 etc., cut by igneous dikes, also much volcanic tuff and agglomerate. 

 Next follows a series of 12,000 feet of less intensely folded 

 but still metamorphic rocks, quartzites, shales, slates, mica schists, 

 with dikes and interbedded sheets of diorite. A few fossils have 

 been found in the quartzites of this series. The third series has 

 a maximum thickness of 50,000 feet, though usually much less. 



