THE CENTRAL REGION OF NORTH AMERICA. 617 



the east or north-east is mingled. South of the watershed-plateau 

 the third region (that sloping to the Missouri, where it is well 

 sheltered to the north) shows the quartzite drift in even greater 

 purity. Where, however, gaps or lower places in the watershed- 

 plateau occur, incursions of Laurentian rocks and of eastern lime- 

 stones are also found to a greater or less extent. 



The general character of the travelled drift of the third steppe 

 may be seen from its percentage composition, derived in the same 

 way as already shown for the second steppe. 



Laurentian 27*05 



Huronian ? 



Limestone 15*84 



Quartzite Drift 52*10 



Though the percentage of Laurentian material appears nearly the 

 same as before, the much smaller total quantity of drift on this level 

 must be remembered. A mark of interrogation is put after Huro- 

 nian, to indicate that a few specimens of this formation may be pre- 

 sent, but, if so, are undistinguishable from some varieties of the 

 Quartzite drift. The great decrease in limestone is at once seen ; 

 and even the percentage here given includes some specimens of 

 Rocky-Mountain limestone which has travelled eastward with the 

 Quartzite drift. The limestones of the flanks of the Laurentian 

 were probably completely submerged ere the water reached the level 

 of the third steppe. Quartzite and similar rocks now form over half 

 of the entire travelled portion of the drift deposit. 



Some of the lower parts of this steppe show thick deposits of true 

 till or boulder-clay, which holds in a hard yellowish sandy matrix 

 well glaciated stones, both from the mountains and from the east, 

 and also a great quantity of debris from the softer underlying beds, 

 among which are fragments of lignite from the Tertiary. These 

 deposits of till, though generally massive and weathering into rudely 

 columnar forms in perpendicular banks, often show traces of bedding 

 and arrangement in water ; and false-bedded sandy masses are found 

 abruptly cut off above the confused bouldery clay. The shingle de- 

 posits of the higher levels may perhaps be formed partly from the 

 rearrangement of this material ; they are at least superior to it. 



The width of the third steppe, on the line, is about 450 miles ; but 

 it narrows rapidly northward. Its surface is more diversified and 

 worn than that of either of the other prairie -levels ; and the occur- 

 rence and features of the drift are less constant. Following it west- 

 ward, and in the main slowly rising, Laurentian and Eastern lime- 

 stone boulders continue to occur to within about 25 miles of the 

 base of the Rocky Mountains, at a height of about 4200 feet. The 

 distance of these travelled blocks from the nearest part of the Lauren- 

 tian region is over 700 miles. Beyond this point eastern and north- 

 ern rocks were not found : but that the depression of the continent 

 ceased here cannot be argued from this fact ; for by this time the 

 whole of the Laurentian highlands would be submerged. 



On the higher prairie; sloping up towards the mountains, the 



