POTSDAM SANDSTONE—CONGLOMERATE. 57 
thin out rapidly toward the west, and on the Mississippi do not include a thousand feet of 
strata. 
An equally marked change of lithological character is perceptible, following the same line of 
observation. In New York the ‘‘Old Red Sandstone’’ formation includes thick beds of sandstone 
of typical character. In Ohio these have disappeared, and the Devonian strata consist of sandy 
or carbonaceous shales and limestones; further west little but the limestones remain. The 
structure of the Black Hills, Nebraska, according to Dr. Hayden, (Proceedings Academy Natural 
Sciences, Philadelphia, June, 1858) seems to afford strong presumptive evidence of the Silurian 
age of the sandstones in question. Dr. Hayden found the crystalline igneous and metamorphic 
rocks, forming the core of the Black Hills, overlaid by upheaved sediments, of which the lowest 
is a coarse deep red sandstone containing Lingula, Obolus, etc., evidently the Potsdam. Above 
this are, partially metamorphosed, silicious limestones, &c., and then the Carboniferous lime- 
GRANITE PINNACLES, CANON OF VER. 
stone.. This is the nearest point to the exposures in the cafion of the Colorado, where strata 
of any considerable thickness have been found below the Carboniferous formation, and the 
section given by Dr. Hayden exhibits a striking paralellism to that under consideration. 
The conglomerate which underlies or rather forms the base of the red sandstones, to any one 
who has seen the Potsdam of Lake Superior in place, will seem an old acquaintance. It is but 
a portion of the sandstone which includes pebbles of quartz, jasper, &c., very possibly derived 
from the attrition of the granite on which it rests. It is much harder than any of the more 
recent conglomerates met with on our route, resembling more the dense semi-crystalline con- 
glomerate in the vicinity of the trap on Keewanaw Point. The surface of the granite beneath 
is, in many places, planed off, as though by the action of currents of water flowing over it 
before the deposition of the stratified rocks. 
8 L 
