ON THE MINNESOTA RIVER. 485 
At the head of a small island, about three miles beyond, the same rocks were 
again observed jutting out at intervals from the slope of a hill on the left bank of 
the river. 
After proceeding about three miles further, we reached Mankassa, or White Earth 
Bluff, situated on the east side of the Minnesota. It is a perpendicular escarpment 
seventy feet high, fifty-five feet of which is white and brown sandstone, and fifteen 
feet of rather thin-bedded magnesian limestone; the two formations being sepa- 
rated by a thin seam of green siliceous earth. There is, therefore, a rise in the 
strata of about thirty feet between this point and that of the last section. 
From the summit of the bluff a singular view is presented. A nearly level 
plain, about a mile in width, is seen extending for several miles in a direction 
nearly parallel with the river. This plain is bounded on the east by a beautiful 
terrace of table land, about eighty feet high, from which stretches a fine undulating 
prairie towards the interior. Rising from the surface of the lower plain, large 
masses of magnesian limestone and immense numbers of boulders of granite and 
trap can be seen in all directions. The magnesian limestone occurs in isolated 
masses only, from eight to ten feet high, and from forty to fifty feet in circumference, 
having at first sight all the appearance of erratic blocks; yet a close inspection 
shows that in every instance they are composed of horizontal layers, varying from 
half an inch to several inches in thickness, arranged as in the original stratification ; 
thus proving clearly that they are not transported masses, like the associated igneous 
and metamorphic boulders, as has been supposed, but are, in fact, the harder por- 
tions of the original sedimentary strata, which have resisted the influence of currents 
and other denuding forces, whilst the softer materials have been swept away by 
these agencies. 
Two miles above White Earth Bluff, on the left bank of the river, is a hill two 
hundred and fifty feet high, at the base of which twenty-five feet of sandstone 
(F. 1) is exposed. The remaining portion of the hill is covered with soil and 
densely wooded. Two miles further, the same rock is again exposed, in thick beds, 
on the opposite bank of the river, where its thickness is forty feet. Three miles 
higher, magnesian limestone (F. 2) projects at intervals from the base to the top of 
a hill, seventy feet high, presenting an abrupt slope towards the river. Near the 
summit of the hill the rock is in nearly horizontal layers, but towards the foot it is 
in detached masses, without any appearance of regular bedding, so that it is pro- 
bable these have fallen from above. 
Four to five miles above this place, and by the course of the river, about twenty 
miles above Traverse des Sioux, a good section of sandstone, capped with magnesian 
limestone, is exposed on the east bank of the river, forming a perpendicular escarp- 
ment seventy feet high. The sandstone extends to the height of fifty feet, and 
supports twenty feet of magnesian limestone. 
On the table land above the bluff we observed a great many large boulders, some 
of them angular and others rounded ; one of these blocks, consisting of a fine-grained 
variety of granite, measured thirty-five feet in circumference, and projected more 
than seven feet above the prairie-level. Here an ancient channel of the St. Peter’s, 
half a mile from the present one, was measured, and found to be seven hundred and 
