1889.] NEW YORK ACADEMY OF SCIENCES. 1& 



There are several points related further to the views of this 

 paper, which ma3Miere be briefly presented, 



(I.) As to the "origin and structure of faults. — Prof. LeCoute 

 has recently announced, before the Society of American Geolo- 

 gists, a theory as to the origin of normal faults, — a theory which 

 is ingenious, but not to my mind wholly satisfactory. He con- 

 ceives an upswelling of the crust over a large area, and the ulti- 

 mate fracture of it into a number of blocks or pieces having a 

 rhomboidal vertical section. The fracturing allows the escape 

 of igneous or gaseous matter, which by its accumulation or 

 '' intumescence '' has caused the elevation of the region. The 

 blocks then settle down somewhat irregularly, tending by gra- 

 vity to tip in such a manner as to bring about the position 

 found in normal faults, i.e., with " the hade toward the down- 

 throw." The view is ingenious and simple ; but it seems as 

 though the process would be subject to so many complications 

 that it could scarcely result in a general law, especially in the pre- 

 sence of the tendency to compression and contraction in the 

 crust, which would be apt to produce an opposite result. 



But if faults arise in the manner of marginal crevasses, i.e., hj 

 rupture across lines of permanent crust-tension, it is plain that 

 the law of normal faulting must follow of necessity. Keversed 

 faults and normal faults may then appear as the consequence re- 

 spectively of compression and of extension. The course of the 

 former should {originally) have been eoclinal, and of the latter 

 hesperoclinal. The association of reversed faults with folded 

 strata is already well known, and indeed seems inevitable. Where 

 faulting has occurred in overturn folds, caused by lateral thrust, 

 the hanging wall must properly override the foot-wall, — the 

 latter having the downthrow and the former the upthrow. In 

 this case, the readjustment of parts is such as to show actual hori- 

 zontal compression, as in the folds and overturns associated with 

 it. But in the case of a normal fault, the readjustment is one 

 of horizontal extension, and would certainly indicate as its cause 

 a stretching rather than a crushing force. 



(II.) The earth's velocity of rotation has thus far been referred 

 to as though constant in rate. If, on tlie other hand, as gen- 

 erally believed, it has been subject to any variation in the past, 

 important results must follow as to the liorizontal strains. In 

 the process of contraction, the reduction of volume would natu- 

 rally produce increased rapidity of rotation, from the tendency 

 to the persistence of inertial velocity. If so, the result would 

 be to maintain and intensify the action already suggested. If, 

 on the other hand, by tidal friction or lunar retardation, the 

 rate of axial rotation has tended to diminish, any balance al- 

 ready established in the strains of the crust would be disar- 



