CLIMATES OF GEOLOGIC TIME—SCHUCHERT. 293 
climates. We can therefore say that the temperatures of air and 
water had been mild to warm throughout the world since the begin- 
ning of Cambric time, that there was a marked increase of warmth 
in the Upper Cambric, and that these conditions were maintained 
throughout the Ordovicic and the earlier half of the Siluric, since 
shallow-water corals, reef limestones, and very thick dolomites of 
Siluric time are as common, in arctic America as in the lower latitudes 
of the United States or Europe. 
The Siluric closed with an epoch of sea withdrawal and North 
America was again arid, for now red shales, gypsum, thick beds of 
salt, and great flats of sun-cracked water limestone were the dominant 
deposits of the vanishing seas. The marine faunas were as a rule 
secant and the individuals generally under the average size. In 
North America no marked mountain making was in progress, but 
all along western Europe, from Ireland and Scotland across Norway 
into far Spitzbergen, the Caledonian Mountains were rising. In 
eastern and northern Maine throughout Middle and Upper Siluric 
time there were active volcanoes of the explosive type, for here 
occur vast deposits of ash. 
Devonic.—In the succeeding Lower Devonic time the Caledonian 
intermontane valleys of Scotland and north to at least southern 
Norway were filling with the Old Red sandstone deposits of a more 
or less arid climate. On the other hand, the invading seas of northern 
Europe were small indeed, and their deposits essentially sandstones 
or sandy shales, but in southern Europe and North America, where 
the invasions were also small and restricted to the margin of the 
continent, the deposits were either limestones or calcareous shales. 
The life of these waters was quite different from that of the earlier 
and Middle Siluric, and entire stocks had been blotted out in later 
Siluric time, as is seen best among the graptolites, crinids, brachio- 
pods, and trilobites, while new ones appeared, as the goniatites, 
dipnoans or lung fishes, sharks, and the terrible armored marine 
lung fishes, the arthrodires. 
From this evidence we may conclude that the early Paleozoic mild 
climates were considerably reduced in temperature toward the close 
of the Siluric and that even local glaciation may have been present. 
Refrigeration may have been greatest in the Southern Hemisphere, 
where the marine formations of Devonic time are coarse in character 
and, in Africa, of very limited extent. Corals were scarce or absent 
here, and in South Africa the glacial deposits of the Table Mountain 
series may be of late Siluric age; if so, they harmonize with the Cale- 
donian period of mountain making in the Northern Hemisphere. 
Warmer conditions again prevailed in the latter hemisphere early in 
Middle Devonic times, for coral reefs, limestones, and a highly varied 
marine life with pteropod accumulations were of wide distribution. 
