568 PC. (CHAMBERLIN, 
(6) the close of the Jurassic and the opening of the Lower Cre- 
taceous, and (7) the transition period between the Cretaceous 
and the -EKocene. “In these” periods, there are jevadencesiot 
declared intensifications of climatic influence expressed in wide- 
spread and thick deposits of salt and gypsum and in great series 
of red sandstones and marls, and, in the two most notable cases, 
by pronounced glaciation in middle and low latitudes. 
On the other hand, it regards the following as periods of sea 
extension attended by the active freeing of carbon dioxide 
through the agency of prolific lime-secreting life, as hereafter 
set forth: (1) the middle Ordovician, (2) the middle Silurian, 
(3) the sub-Carboniferous, (4) the late Jurassic, (5) the Upper 
Cretaceous, and (6), less notable, the later Eocene and earlier 
Miocene. During these periods there is evidence of extensive 
limestone deposition spreading out widely on the continental 
platforms, attended by very mild and equable climates very 
nearly uniform for all latitudes. 
SOURCES OF TEMPORARY LOSS AND GAIN 
The discussion has thus far taken note of the original carbo- 
nation of the silicates of crystalline rocks only. These crystal- 
line rocks, according to Dr. Tillo,t occupy something over 20 
per cent. of the surface of the land. There remains nearly 80 
per cent. occupied by secondary rocks which now claim attention. 
Sources of temporary loss—The function of the secondary deposits. 
—To a large extent the material of the secondary deposits 
underwent chemical decomposition and carbonation preliminary 
to its deposition, indeed as a prerequisite to its derivation. In 
so far as this process was incomplete in the earlier stages, it was 
continued during any subsequent state of exposure, but this 
action belongs under the preceding head of original carbonation. 
In the erosion of tracts of secondary rocks the limestones 
and dolomites are dissolved and carried down to the sea essen- 
tially as bicarbonates. In the strata they existed as monocar- 
bonates. Their solution involves the taking up of a second 
* Berghaus Atlas. 
