136 RECONNAISSANCE FROM CARROLL, MONTANA, 
twenty-five miles from the river and about ten miles from the north limit of the Judith Mountains, 
where the surface-drift changes its character and commences to be made up entirely of trachyte 
from Cone Butte and the neighboring hills. To the north, the limit is not so distinctly marked ; but 
it is reached within twenty miles of the river, where the trachyte of the Little Rocky Mountains 
takes its place. This drift is divided into two classes quite distinct from each other: first, we 
have the rounded pebbles, very uniformly scattered over the surface of the prairie; and, second, 
the large, angular blocks distributed here and there at random. The pebbles are generally small, 
sometimes several inches in diameter, but more frequently much less. They are flattened, quite 
smooth, and in fact bear much the appearance of common stream-pebbles; they are almost never 
glaciated. They show, however, the marks of the force of attrition by which they have been 
smoothed into shape, for the surface-layer of those of uniform texture is curiously marked with 
semicircular cracks, due to the constant blows which they have received against each other, giving 
them often quite an ornamented appearance. The material of the pebbles is 90 per cent. quartzite, 
generally yellow, sometimes dull red (jasper), and also rarely dark-colored. The remaining 10 per 
cent. is made up of material so heterogeneous that a catalogue of the varieties would be more 
curious than valuable; pieces of fossil wood, however, must be mentioned. As has been stated, 
the deposits are superficial in all cases. The material composing the drift of the second class is 
very generally a bright-red syenite; this forms masses sometimes three or four feet’ in thickness, 
but averaging about 18 inches. Next in importance is a similar rock, in which the place of the 
hornblende is taken mostly by black mica; still again, there are masses of black hornblende rock, 
a grayish syenite, but very little true granite. All these have a very Archean look. Masses of 
setui-crystalline limestone also occur, though not frequently. These blocks, as has been stated, are 
uniformly angular, showing little trace of wear. They are less uniformly distributed than the 
pebbles. 
The source of these drift masses can hardly be held in doubt. Confined, as they are, to the 
Missouri Valley, they make it almost certain that they have been brought by running water in the 
direction of the present stream. In the flood which followed the melting of the ice, which, to a 
greater or less extent, doubtless covered the higher mountains, and at a time when the land is sup- 
posed to have been depressed, the waters may well have spread over a width of forty miles, cover- 
ing the now so nearly level prairie, and could readily have transported the smaller washed pebbles. 
The large blocks evidently demand stronger agencies, and it is difficult to make any other suppo- 
sition than that they have been carried by floating ice brought from the westward, from the high 
mountains which form the main divide of the Rocky Mountains, in which the red feldspar-syenites 
and the quartzites must have a large development. This would account for their not being rolled 
bowlders. To the same time of glacial floods belong the formations of the terraces seen ; especially 
those at the Little Rocky Mountains and Judith Mountains. 
Our opportunities for making observations above and below Carroll on the river were exceed- 
ingly limited. Masses of a syenitic rock were observed, here and there, down the river, prominent 
at the mouth of the Musselshell River, and again at Fort Peck. Running notes from the steamboat- 
deck have little value, and not much ean be based upon them. Far down the Missouri, near Bis- 
marek, eight hundred miles from Carroll, the drift bowlders are numerous, and the quaternary sands 
form deep stratified deposits. These phenomena, however, join on to those which are observed more 
and more decidedly to the eastward, and the source of which is to be found to the northeast. West 
of Carroll, near the mouth of the Judith River, the drift just described was not observed. This 
evidence is negative merely, since, if once deposited as below, it can easily be imagined that 
subsequent denedation has obscured it. 
It is interesting to note, in connection with the facts stated in regard to the drift from the 
westward, the extended and careful observations of a similar character, made at many different 
points, by Mr. G. M. Dawson, F. G.8., and described in the “ Geology and Resources of the Region 
jn the Vicinity of the Forty-ninth Parallel,” Montreal, 1875. 
If the report in question be consulted, a full description of these interesting facts will be found. 
It is sufficient for our purposes to call attention to the great prevalence of the quartzite drift over 
the prairie far to the north of the Missouri. The general character of this drift was much the same 
as that found by us, and it was also referred to the Rocky Mountains as its source. 
