ON THE nORIZOXTAL MOVEMENTS OF EOCKS, ETC. 783 



44. On the Horizontal Movements of Rocks, and the Relation of 

 these Movements to the Formation of Dykes and Faults and to 

 Denudation and the Thickening of Strata. By William 

 Barlow, Esq., F.G.S. (Read April 25, 1888.) 



I propose to call attention to some horizontal movements of rocks 

 occasioned by gravitation, the importance of which has, I believe, 

 been almost entirely overlooked, and shall try to show that the great 

 forces of denudation in many cases owe much of their power to 

 Assuring and dislocation produced by these movements, also that to 

 these movements is to be referred the production of dykes and 

 faults. 



In the Grand Canon District of the American Union a wide 

 expanse of elevated horizontal strata, some thousands of square 

 miles in extent, has been denuded in such a manner as to display 

 a succession of huge terraces or steps of successive strata, each 

 terrace being terminated by a sinuous line of cliffs or abrupt slopes. 

 Between the succeeding escarpments the strata dip slightly from 

 the crest of the one below to the foot of the next above. In the 

 median parts of any given terrace the strata are very nearly hori- 

 zontal and have inclinations scarcely exceeding one degree ; hut as 

 ive approach the escarpment of tlie next higher terrace, the inclination 

 increases to three or four degrees^ becoming a maccimum at the base 

 of this IV all. 



The cumulative effect of the slight dip thus displayed, which for 

 the most part has a northerly direction, is that the top of a certain 

 stratum, the Carboniferous, is more than 8000 feet lower at the 

 north, below the topmost terrace, than at the south, where it comes 

 to the surface and forms a wide plateau, the lowest terrace. The 

 difference in altitude between the highest and the lowest terrace is 

 several thousand feet *. 



In the same district of horizontal strata the forces of denudation 

 have removed the upper strata to a great depth over a large area- of 

 elliptical form, producing in this way a great hollow, many miles in 

 diameter, enclosed by cliffs and known as '^ The San Rafael Swell." 

 Here also there are indications of a slight elevation of the unloaded 

 strata M'ithin the denuded space as compared with the continuation 

 of the same strata where they are heavily loaded beneath the sur- 

 rounding cliffs t. 



It has been suggested by Mr. Dutton that the phenomenon referred 

 to is analogous to the action of creeping in deep mines ; and Mr. 

 Clarence King, in reference to the subsidence of strata in the same 

 locality, makes a similar suggestion +. 



* ' Tertiary Hist, of the Grand Caiion District,' C. E. Dutton, pp. 47, 70. 



t ' Geology of the High Plateaus of Utah,' Dutton, pp. 18-21. 



X U. S. Geological Exploration of the 40th Parallel. — I. Systematic Geology. 



3e2 



