82 2 G. K. GILBERT 



extreme slowness to the ordinary processes of disintegration. 

 It results that in regions of rapid degradation laccolites usually 

 survive all associated rock-masses and find topographic expres- 

 sion in steep-sided mountains or buttes. In these respects the 

 rock of the Twin Butte locality is strongly contrasted. It 

 weathers more easily than the associated sandstone, so that dikes 

 traversing sandstone outcrops are sometimes not easy to trace ; 

 and it is scarcely more durable than the associated shales. The 

 dikes do indeed stand prominent above shale areas, but this is 

 occasioned largely by the baking of the shales along the planes 

 of contact, and also in part by an alteration of undetermined 

 character which in some instances affects the dike rock for a few 

 inches from the planes of contact. Where the laccolites are in 

 contact with shales the latter are modified for many feet or 

 yards, being rendered much more durable than the igneous 

 rock. It results that the exposed parts of laccolites, being 

 weaker than all the associated sedimentaries, are characterized 

 topographically by valleys. 



I. C. Russell, reasoning from the dominance of acid rocks 

 among laccolites, the frequent occurrence of basic rocks as thin 

 sheets, and the viscosity of acid magmas as compared to basic 

 at the same temperatures, has recently suggested^ that great 

 viscosity is an essential condition of the production of thick 

 intrusive lenses". As this theory encounters serious diflficulty 

 in the new data from Montana and southeastern Colorado, I 

 venture an alternative suggestion involving a different point of 

 view. The topographic features associated with the weak basic 

 rocks at Twin Butte are inconspicuous, whereas those of the 

 resistant porphyrites of the Elk and Henry mountains are bold 

 and striking. Is it not possible that the basic rocks are really 

 well represented in laccolites, but have as yet received little 

 notice because in the gradual development of the subject the 

 more salient features have first caught the eye ? 



G. K. Gilbert. 

 'Jour. Geol., Vol. IV, pp. 179-180, 1896. 



