792 ME. W. BAELOW OK THE HORIZONTAL 



mountains which I attribute to horizontal movements such as I have 

 above treated of. 



The very shape of the lava-deposits formed within the strata 

 suggests that they have spread by their own weight and the weight 

 of the superimposed crust. Circular or elliptical on plan, they are 

 nearly flat in the middle, and curve down more and more rapidly 

 towards the circumference*. Thus they have much the form taken 

 by a drop of viscous fluid placed upon a level surfacef. 



Dykes rise from the upper surfaces of the deposits. These are 

 largest and most numerous about the centre, and the largest of them 

 mostly radiate from the centre outward. Where numerous they 

 reticulate %. 



This predominance of the dykes in the axial regions of the lava 

 masses, a well-known phenomenon in volcanic mountains, is possibly 

 due to the crust experiencing most strain where most motion of the 

 molten matter beneath takes place, the spread of the molten matter 

 facilitating the spread and rupture of the crust resting upon it. 



Another phenomenon pointing in the same direction is that faults 

 are present in some cases which are subordinate -phenoraena of the 

 uplift. They are restricted to its central portion^ and never occur so 

 far from the centre as the zone of maximum dip of the domed strata. 

 The strata of the upper part of the arch are in this case divided into 

 a number of prismoid blocks, which stand at slightly different levels. 

 All or nearly all of the fault-planes are occupied by dykes of 

 trachyte §. 



Further, the horizontal movement of some layers of strata on 

 others is proved by the fact that in a number of instances the dykes 

 are as even upon their upper surfaces as an artificial stone wall, the 

 flat top of the dyke butting against an unbroken stratum of rock 

 which bridges across it, and being parallel to the bedding of the 

 enclosing strata. In one case a converse phenomenon is seen. A 

 great dyke forms the crest of a ridge for half a mile, its base being 

 buried in sandstone, and at the end of the ridge the strata are seen 

 to be continuous beneath the dyke||. 



Then there is a fact which we may refer to the presence of joints 

 or close flssures caused by small horizontal motions of the parts of 

 the strata. The denudation has been far greater where the strata are 

 uplifted to form the mountains than in the region around. Thus, 

 while from the base of the arch of one of the mountains 3500 feet of 

 the Cretaceous and from 500 to 1500 feet of the Jura-Trias series 

 have been removed, from the summit of the arch more than 2500 feet 

 of the latter have disappeared. In cases where the lava deposits are 

 so deep that the denudation has not laid them bare, the arched 

 sedimentary rocks of the uplift have often been eroded down to 



* ' Geology- of the Henry Mountains,' p. 55. t ■^«^- PP- 20, 23. 



\ Many mountain uplifts have much this form, e.g. the Uinta Mountains, 

 the Kaibab Plateau, and the Black Hills of Dakota. See Geology of the Black 

 Hills of Dakota,' p. 207. 



§ ' Geology of the Henry Mountains,' p. 23. 



II Ibid. pp. 28, 34. 



