CLIMATES OF GEOLOGIC TIME—SCHUCHERT. 283 
CAMBRIC GLACIATION. 
Unmistakable tillites, thought to be of earliest Cambric age, have 
been described by Howchin and David from southern Australia and 
by Wills and Blackwelder from China. In both cases the evidence 
as to age is open to question, as the tillites are either sharply sepa- 
rated from the overlying Cambric deposits or these strata have no 
fossils to fix their age, thus leading to the inference that the tillites 
are more probably of late Proterozoic time. In arctic Norway occur 
other tillites at the base of the thick Gaisa formation. These deposits 
also were formerly regarded as of Paleozoic age, but Norwegian 
geologists now refer them to the Proterozoic. All of these tillites 
are best referred to the vast era previous to the Cambric period. 
LATEST PROTEROZOIC GLACIATION. 
Australia.—In southern Australia, conformably beneath marine 
and fossiliferous Lower Cambric strata but sharply separated from 
them, occur tillites of wide distribution. They extend from 20 miles ° 
south of Adelaide to 440 miles north of the same city, with an east- 
and-west spread of 200 miles. Bowlder clay has also been discov- 
ered on the west coast of Tasmania. The tillites range in thickness 
from about 600 to 1,500 feet and occur at the top of a vast pile of 
conglomerates, grits, feldspathic quartzites, slates, and phyllites, 
whose exact age is unknown because as yet no fossils have been 
discovered in them. (See fig. 3.) 
According to Howchin, the tillite consists ‘‘mainly of a ground- 
mass of unstratified, indurated mudstone, more or less gritty, and 
carrying angular, subangular, and rounded bowlders (up to 11 feet 
in diameter), which are distributed confusedly through the mass. 
It is in every respect a characteristic till” (1908: 239). The first 
scratched bowlders were observed in 1901, and now they are known 
by the “thousands” (David). They range in size up to about 10 
feet long. So far no striated underground or glaciated floor has 
been discovered, and both Howchin and David hold that the tillite 
was formed at or near sea level in fresh or brackish water with floating 
icebergs. The rocks of the tills, David thinks, came from the south. 
The tillite is now found from below sea level to about 1,000 feet 
above the sea. These tillites and all of the enormous mass of coarse 
deposits below them, which is at least several miles thick, the Aus- 
tralian geologists regard as of Lower Cambric age, because overlying 
them occur fossils of this time. The contact between the tillite 
and the marine Cambric is always a sharp one, leading to the inference 
that the sea of this time transgressed over an old flat land. Under 
these circumstances deposition was not continuous, for the geologic 
