carbon dioxide concentrations in the Precambrian probably account for the
prevalence of dolomite over limestone. These minerals appear to be mainly
chemical precipitates and not of biological origin.
In the Mackenzie Mountains of northwest Canada, dolomite deposits
range up to 6,500 feet thick. In the Alps, massive chunks of dolomite soar sky-
ward. Carbonate rocks such as limestone and chalk, produced chiefly by
organic processes involving shells and skeletons of simple organisms, became
more common in the late Proterozoic beginning about 700 million years ago.
In contrast, they were relatively rare in the Archean due to the scarcity of
lime-secreting organisms.
The continents of the Proterozoic comprised Archean cratons. The
North American continent assembled from seven cratons around 2 billion
years ago, making it the oldest continent.The cratons welded into what is now
central Canada and north-central United States. Meanwhile, continental col-
lisions continued to add a large area of new crust to the growing proto–North
American continent (Fig. 43). At Cape Smith on Hudson Bay is a 2-billion-
year-old slice of oceanic crust that was accreted onto the land, indicating that
Figure 42Ladore
Canyon and the Green
River in the Uinta
Mountains, Summit
County, Utah.
(Photo by W. R. Hansen,
courtesy USGS)PROTEROZOIC METAZOANS