Origin of Jfoi',,f(i!n Rdnfjr^. — Recide. 279 
continuously connected, though this has not yet been proved. 
The same formations extend far to the eastward of the Rocky 
mountains and the Andes, and most proljabh' to the westward 
under the Pacific ocean. 
The greatest ranges of the world have been elaltorated in Cre- 
taceous or Tertiary times, and the connection between sedimenta- 
tion and upheaval is here most striking. 
SedimentH out of irJiich Moinitoui-Riingf^s Jifivr been el aho rated icerc 
laid do ic II in Basins or Troiiylis fornwd hi/ tin: handing of tin- 
eartJis crust. 
The thickness of the rocks, mostly conformable, composing- 
some great mountain-ranges has been estimated by competent 
geologists at from eight to ten miles. The bulk of the rocks, as 
judged by their constitution, are usually considered by geologists 
to largely indicate either a moderate depth of water or actual 
shallow^ conditions. These rocks are intercalated with others ex- 
hibiting signs of a more oceanic origin. All the mountain - 
ranges mentioned may be pointed to in illustration of this state- 
ment. There is thus evidence that regional fluctuations of level 
ill the earth's crust have taken place on a large scale often suc- 
ceeded by. as in the case of the Coal-measures, continued down- 
ward subsidence comljined with shallow-water conditions. 
It is evident, from these facts, that the great earth-troughs, 
in which these materials for mountain-building were accumulated, 
were in some cases, on the final completion of sedimentation, 
double the depth of the deepest known oceanic troughs, which do 
not reach more than five miles. 
Considering that there is a strong devel<jpment of Cretaceous 
and Tertiary rocks extending along the western coast of North 
and South America, it is seen that these operations have there 
been carried on on an unusual scale. Deposit and alteration of 
level, elevation and subsidence, but preponderantly subsidence, 
pi'ogressed for an immcnsi' length of geological time in these 
areas, occupying not a mean portion of the eartii's history. 
It is not. however, to be assumed that this was a continuous 
trough at any one time, rather that it consisted of a series of con- 
nected basins which underwent indej)endent dianges of level, the 
area being part of the time low-lying land interchanging with con- 
ditions of suVjmergence. 
