290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
effect has been clearly traced back to 1750, or to the time of the earliest reliable 
records. Hence itis safe to say that such a relation between volcanic dust in the upper 
atmosphere and average temperatures of the lower atmosphere has always obtained 
and therefore that volcanic dust must have been a factor, possibly a very important 
one, in the production of many, perhaps all, past climatic changes. 
The intensity of the solar radiation at the surface of the earth depends upon not 
only the dustiness of the earth’s atmosphere but also upon the dustiness, and of course 
the temperature, of the solar atmosphere. Obviously dust in the sun’s envelope 
must more or less shut in solar radiation just as and in the same manner that dust in 
the earth’s envelope shuts it out. Hence it follows that when this dust is greatest, 
other things being equal, the output of solar energy will be least, and that when the 
dust is least, other things being equal, the output of energy will be greatest. Not 
only may the intensity of the emitted radiation vary because of changes in the trans- 
parency of the solar atmosphere but also because of any variations in the temperature 
of the effective solar surface which, it would seem, might well be hottest when most 
agitated, or at the times of spot maxima, and coolest when most quiescent, or at the 
times of spot minima. 
BIOLOGIC EVIDENCE. 
In the previous pages there has been presented the evidence for 
cold climates during geologic times as furnished by the presence of 
the various tillites. This presentation has also been made from the 
standpoint of discovery of the tillites, which in general is in harmony 
with geologic chronology, i. e., the youngest tillites were the first to 
be observed, while the most ancient one has been discovered recently. 
Variability of climate is also to be observed in the succession of 
plants and animals as recorded in the fossils of the sedimentary rocks. 
In this study we are guided by the distribution of living organisms 
and the postulate that temperature conditions have always operated 
very much as they do now upon the living things of the land and 
waters. In presenting this biologic evidence we shall, however, 
begin at the beginning of geologic time and trace it to modern days, 
for the reason that life has constantly varied and evolved from the 
more simple to the more complex organisms. 
Proterozoic._—The first era known to us with sedimentary forma- 
tions that are not greatly altered is the Proterozoic, a time of enor- 
mous duration, so long indeed that some geologists do not hesitate 
to say that it endured as long as all subsequent time. These rocks 
are best known and occur most extensively over the southern half 
of the great area of 2,000,000 square miles covered by the Canadian 
shield. There were at least four cycles of rock making, each one of 
which, in the area just north of the Great Lakes and the St. Lawrence 
River, was separated from the next by a period of mountain making. 
These mountains were domed or batholithic masses of vertical uplift 
due to vast bodies of deep-seated granitic magmas rising beneath 
and into the sediments. Ih the Grenville area of Canada, Adams 
and Barlow (1910) tell us that the total thickness of the pre- 
Proterozoic rocks alone is 94,406 feet, or nearly 18 miles. Of this 
