200 New York State Museum 



Three divisions of time are recognized here, Early, Mid- 

 dle and Late Proterozoic. Proterozoic rocks on the whole 

 are only slightly deformed, and therefore are sharply set 

 off from the underlying strongly folded Archeozoic series 

 with its many igneous intrusions by this condition of the 

 strata as well as the widespread unconformity. They are 

 also mainly sedimentary (volcanic rocks are subordinate) 

 and when not too greatly metamorphosed are similar in 

 all essential respects to the rocks of later eras. The Early 

 Proterozoic rocks (Sudburian series) overlie the Lau- 

 rentian peneplane from Lake Huron north to Sudbury, 

 Ontario, and are not known elsewhere. They have a 

 total known thickness of about 20,000 feet, consisting of 

 white, cross-bedded quartzites with interbedded shales 

 (15,000 feet) and below this cross-bedded arkoses and 

 impure sandstones or graywackes (5000 feet) and some- 

 times a basal conglomerate. The upper deposits proba- 

 bly represent great alluvial or delta deposits; the lower 

 are found in close association with the Laurentian gran- 

 ites and were formed through the disintegration of these 

 either under desert conditions or, more probably, in a 

 cool moist climate. These lowest Proterozoic beds are 

 so little altered when not intruded by later igneous erup- 

 tions that original structures such as bedding, cross-bed- 

 ding and ripple marks may be seen. They are much 

 metamorphosed where intruded by the granites and are 

 also deformed, thus being sharply set off from the Huron- 

 ian series above, which are nearly flat or only gently 

 folded. The Early Proterozoic is marked at the end by 

 a period of mountain making followed by an erosion 

 interval. 



The Middle Proterozoic rocks rest unconformably 

 upon the Lower Proterozoic and the unconformity is con- 



