204 New York State Museum 



indicates that this area suffered a period of glaciation 

 at this time such as our Great Ice Age. Tillites of Pro- 

 terozoic age, sometimes resting upon a striated rock 

 pavement, have now been discovered on the north coast 

 of Norway, in eastern China, India, South Australia and 

 probably South Africa. Some investigators believe that 

 some of these ice ages belong to the Cambrian and that 

 the Cambrian began with a glacial climate but this has 

 not yet been established. For long stretches of time the 

 climate of the Precambrian must have been mild and 

 fairly uniform over all the world, since the thick lime- 

 stone deposits, the algae concretions and beds of iron all 

 indicate mild conditions at the time of their deposition. 

 The glacial periods appear to have been of short dura- 

 tion, geologically speaking, and to have taken place at 

 times when the land stood highest. 



The Grenville Series of New York 

 The Grenville series are a vast succession of sedimen- 

 tary rocks, essentially calcareous, which due to meta- 

 morphism have become crystalline limestones, gneisses, 

 hornblende schists (amphibolite) and quartzites. They 

 are not only the oldest rocks in New York State but are 

 among the oldest rocks of the earth. These beds are 

 generally believed to belong to the Archeozoic era and 

 constitute the thickest known series of Archeozoic strata, 

 though more recently some geologists have come to the 

 belief that they are of Proterozoic (Huronian) age. The 

 Grenville series was first described by Logan and named 

 after the township of Grenville. These strata occur in 

 Ontario, in the region north of Lake Ontario and east of 

 Lake Huron and it has been estimated that here there 

 is a total thickness of 94,000 feet (18 miles) of which 



