GEOLOGY. 237 



The most notable Nortli American advances of recent years in the 

 observation and interpretation of the resnlts of diastatic movement 

 rehite to faultitij;. Two genetic classes of faults have long been recog- 

 nized — i. e., normal faults, in which the hade (or inclination of the plane 

 of fracture) is toward the thrown side, and reversed or overthrust faults, 

 in which the hade is toward the heaved side; and in general the normal 

 faults have been attributed to stresses not accompanied b}' horizontal 

 compression, and the overthrust faults primarily to horizontal com- 

 pressioi. 



A few years ago Archibald Geikie and his collaborators upon the 

 Geological Survey of Great Britain concluded that the peculiar struct- 

 ure of the Scottish Highlands is determined by overthrust faulting 

 iiI)on a grand scale — older strata being pushed over newer, sometimes 

 for distances amounting to miles. This conclusion was so novel and 

 striking, and so widely at variance from prevailing opinion, that despite 

 tlie ability of the geologists by whom it was enunciated and the appar- 

 ent conclusiveness of the evidence ui>on which it was based, many con- 

 servative students in this country hesitated to accept it ; yet within 

 the last two years there have been brought to light on this side of the 

 Atlantic almost as striking examples of overthrust faulting as those 

 of the Scottish Highlands. 



During 188G McConuell made an extended exploration of the Kocky 

 Mountains among the passes followed by the Canadian Pacific Rail- 

 road, under the direction and auspices of the Geological Survey of 

 Canada. In the course of this exploration he determined the lindts of 

 a remarkable faulted region, now about 25 miles wide, though a rough 

 estimate places its original width at over 50 miles (the difference indi- 

 cating the amount of compression suffered), in which the faults are 

 generally of the overthrust type. The whole region is broken by a num- 

 ber of parallel, or nearly parallel, longitudinal fractures into a series of 

 oblong blocks, and these are tilt(5d and shoved one over the other until 

 they have taken the form of a westerly dipping compound monocline, 

 rising into a succession of ridges. A section through almost any of 

 tiiese ridges, starting from the west, shows, first, Cretaceous shales folded 

 under older formations, ranging from upper Carboniferous downward 

 through the Devonian and Silurian, and even to the Cambrian. The 

 overthrusts on the south fork of Ghost Kiver reach 3 or 4 miles; and 

 in these, as in some other cases, the rocks thus faulted have been sub- 

 sequently corrugated, and the original fault surface has been Hexed 

 into anticlinals and synclinals, parallel to those of the planes of depo- 

 sition.* 



Kecent studies in the Appalachian region by Willis and other officers 

 of the U.S. Geological Survey have brought to light examples of over- 

 thrust faulting, differing only in degree from those of the Scottish 

 Highlands and the t)anadian Rocky Mountains; and these observa- 



Ept. D, of Aunl. Kpt. Geol. aud Natl. Hist. Cauada for 1886-'87. 



