Geologic Time. — Walcott. 347 
continental platform continued without reversal of the subsidence dur- 
ing Paleozoic time. During Cambrian, and it may be late Algonkian 
time, the extended interior Mississippian region was practically leveled 
by denudation, the eroded material being carried'into the Cordilleran and 
Appalachian seas and, probably, to a sea to the south. 
The sedimentation of the Mississippian area in Paleozoic time be- 
tween the Appalachian and the Cordilleran seas, was small as compared 
to that which accumulated in the latter. In Devonian time there does 
not appear to have been any sedimentation in the western portion of it 
west of the 94th meridan and east of the Cordilleran sea, and it was 
slight in the same interval in the Appalachian sea south of the 37th 
parallel*. There is little if any evidence in the sediments of Paleozoic 
time to show that they were deposited in a deep, open ocean; on the 
contrary, they were largely accumulated in partially enclosed seas or 
mediterraneans and on the borders of the continental plateau. The 
former is particularly true of the sedimentation of the Cordilleran and 
Appalachian seas and the broad Mississippian sea. 
The close of the prolonged period of Paleozoic sedimentation was 
brought about by what Dana has termed the "Appalachian 
revolution." The topography of the continent was more or less 
changed, and the conditions of sedimentation that followed were un- 
like those that preceded. This revolution raised above the sea level a 
considerable portion of the Cordilleran and the Appalachian sea beds 
and also of the Mississippian sea, east of the 96th meridian and north 
of the 34th parallel. In its effect it may be compared to the Algonkian 
revolutiont that preceded the deposition of the Paleozoic sediments. 
With the opening of new conditions the sedimentation of the Mes- 
ozoic time began upon the Atlantic border and over large areas of the 
western half of the continent with the deposit of mechanical sedi- 
ments — sands, silts, etc., — during Jura-Trias time. They are of a char- 
acter that naturally follows a period of disturbance of pre-existing 
conditions and the formation of new basins of deposition with more or 
less elevated adjoining land areas. At its close orographic movements 
affecting the positions of the beds occurred upon the Pacific and At- 
lantic coasts and also, to a more limited degree, throughout the Rocky 
mountain region. This does not appear to have extended over the 
plateau region or the central belt between the 97th and 105th meridians. 
The Cretaceous formations have their greatest development between 
the 97th and 112th meridians in Mexico and the United States, in a 
broad belt which extends from the boundary of the latter to the north- 
west into the British Possessions as far as the 61st parallel. They 
*The non-occurrence of Devonian sediment has not yet been fully explained. It 
has been suggested that the sea beyond the reach of mechanical sedimentation was 
too deep for the deposition of calcareous deposits. It is more probable that the sea 
was shallow and an area of non-deposition, or that its bed was raised to form a low. 
level land surface at a base level of erosion that was subjected to very slight degra- 
dation. 
T'l'he term "revolution" is used to describe the culmination of a loDg 6eries of phe- 
nomena that finally resulted in a distinctly marked epoch in the evolution of the con- 
tinent._ The "Appalachian revolution" began far back in the Paleozoic, and culmi- 
nated in thelater stages of the Carboniferous, and the Algonkian revolution probably 
began far back in Algonkian time. 
