72 PROTOZOIC ROCKS 
that it does not harden very promptly under water. It contains more insoluble 
matter than the best hydraulic limestones.* 
By consulting Dr. Shumard’s section (Sec. 3, 5), the relative thickness of these 
divisions can be seen as they are superimposed on the White Sandstone of F. 2 c. 
Section 5 shows the relative elevation of these fossiliferous limestones above the 
Mississippi, between Red Rock and the Falls of St. Anthony. 
Between New Cave and St. Peter’s, on the Mississippi, in consequence of the 
disintegrating character of the underlying sandstone, the shell limestone has been 
undermined, and, from unequal support, long cleavage-lines have ensued, and the 
strata have sunk in places, giving the appearance of sudden local dips and disturb- 
ances. In the same vicinity, gravel and boulders may be seen,-insinuated beneath 
the overhanging ledges of shell limestone, so as to make it appear, at first sight, as 
if a drift deposit had taken place after the deposition of the sandstone (I. 2 c), and 
before the formation of shell limestone. This phenomenon may, in part, be owing 
to disturbing forces acting from beneath; since, just at this place, there are sudden 
elevations and depressions of the strata in short distances, even to the extent of 
nearly a hundred feet in half a mile. (See Section 5.) 
A more satisfactory explanation of it, however, is to be found in the fact of the 
disintegrating character of the underlying formations, which, partly by atmospheric 
agencies, partly by the action of the river, have crumbled and been swept away, 
leaving cavernous spaces underneath. Into these, gravel and boulders have been 
swept by the floods, or, more probably, conveyed in cakes of ice, charged with 
them, which, during the season of high water, and perhaps at a time when the 
waters flowed at a higher level than they now reach, may have been jammed 
beneath the ledges, and there remained, entangled in the disintegrating rubbish, 
until, thawing, they released their rocky burdens where now they lie. 
Thus, how deceptive soever to a superficial observer the appearance of these acci- 
dental interpolations may be, it is altogether improbable that any drift-formation 
actually supervened between the two formations in question. 
The same formation which constitutes the bluffs at the mouth of the St. Peter’s, 
i.e., the White Sandstone (F. 2 c), and Shell Limestone (F. 3 a), extends up that 
stream at least half a mile, where Dr. Shumard found them in place in the bluffs. 
Beyond that, the strata are concealed from view by drift. The outline of the hills, 
however, is such as induced him to believe that the same beds form their nucleus 
for some miles further, before the lower rocks again reach the surface. 
At the Little Rapids, about forty miles up, they are no longer in place. The 
* An analysis of an earthy limestone between the Upper and Shell Limestone of St. Peter's, by Dr. 
Norwood, gave : 
W: 
ater, . : . ‘ : ‘ : ‘ ; 18 
Insoluble silicates, . : 29 
Carbonate of lime, : 36-4 
Carbonate of magnesia, . ; 0-4 
a oxide of iron, and a trace of manganese, : 12-4 
‘had, 5 ites as EON CoV Sth Beas Pale ane ME ae eR near aD MT 
