Bathyliths in time

The oldest bathyliths of early Precambrian (Archean) age are especially well shown in the Shield areas--Canadian, Fennoscandian, Rhodesian, and Australian. Areally they constitute half to three-quarters or more of the exposed Precambrian terranes of those regions. So far, they have invariably been found to be intrusive into the Precambrian supracrustal rocks. Consequently no field evidence for a primordial granitic shell has yet been found. Originally, as announced by Lawson in 1888, only one period of bathylithic intrusion during Precambrian time, the Laurentian, was recognized in the Canadian Shield. Later, in 1913, a younger period of bathylithic intrusion was recognized by Lawson, which he named Algoman and considered to be epi-Huronian. The volume of granitic rock that invaded the crust during this second period of intrusion was believed by Lawson to be much greater than that during the earlier invasion. This estimate that granite was far more voluminously produced during the second period of intrusion runs counter to the oft-repeated statement that granitic intrusion was specially abundant in earliest geologic time because the crust was thin, hot, and mobile. Absolute age determinations show that this statement has been made for portions of the earth"s crust that differ in age as much as 1000 million years. Lawson"s "Laurentian" granite has an age of 2000 or 2500 millions years, the Laurentian of the type region has an age of 1050 million years, and the epi-Huronian ("Killarnean", according to M. E. Wilson) has an age of 800 million years. Largely owing to determination of absolute ages based on radioactive minerals, at least three more periods of granitic intrusion during Precambrian time are indicated for the Canadian Shield, and more will assuredly be found. The conviction has been steadily strengthening that bathylithic intrusion has been determined by the same laws from the earliest Precambrian time to the present. Most bathyliths are of the orogenic type, generally situated in the axial portions of folded belts of geosynclinal sediments. According to Stille the history of the globe shows that the foldable geosynclinal belts have been diminishing continually in total area during geologic time. Bathylithic intrusion must therefore have followed the same course. On the other hand, Birch concluded from an analysis of the few data available that the indicated rate of intrusion or formation of granite was uniform. The bathyliths of North America were therefore measured by means of a planimeter. The results are tabulated according to ages as Precambrian, Paleozoic, Mesozoic, and Tertiary. They appear to show that rate of intrusion during the Precambrian was roughly the same as that during post-Precambrian time. For shorter periods, however, the rate was highly variable, reaching a maximum during late Mesozoic time. Evidence from the Canadian and other Shield areas indicates that granitic invasion of the crust was small in volume during earliest Precambrian time and increased to a maximum in the Middle Cambrian.