506 WILLIS T. LEE 



destroyed by the existing streams, and a bluff or steep slope 

 formed, the mesa-terrace impresses one as being possibly due to 

 a former cycle of erosion. But where the transition slope is 

 intact and the surface of the mesa-terrace passes gently into that 

 of the flood plain the two are seen to be obviously due to a con- 

 tinuous process of erosion uninterrupted by any notable change 

 in the attitude of the land. 



The substructure of the mesa-terrace is similar to that of 

 the mesas proper, but its surface is more irregular and the cover- 

 ing of fragmental material is not uniform in thickness or in dis- 

 tribution. The mesa-terrace is shown, in part, in the middle 

 foreground of Fig. 2 with the buildings resting upon its surface. 

 The covering of the mesa-terrace differs from that of the mesas, 

 first, in the thinner and more irregular nature of the sheet of 

 debris ; second, in the more rounded character of the fragments ; 

 and third, in the greater content of crystalline material. The 

 debris on the tops of the mesas contains about I per cent, of 

 crystalline material, while the debris on the mesa-terrace is made 

 up of sandstone and crystalline material in varying proportions. 

 The crystalline material of the mesas is chiefly quartz and meta- 

 morphic sandstone, such as is contained in the conglomerate of 

 the Red Beds, from which it very probably came, in the main. 

 The crystalline material on the mesa-terrace is largely granitic, 

 and is usually much water-worn and in an advanced stage of 

 decomposition. 



At still lower levels is found the sheet of debris which is now 

 gathering over the valley bottoms of the existing streams. These 

 streams are forming wide bottom lands' which have a gradient 

 at Boulder of about fifty feet to the mile. In this recent debris the 

 material is imperfectly sorted and is composed largely of 

 crystalline rock .from the mountains. The fragments are well 

 rounded and the granitic constituents are not usually decomposed. 



In these three stages we find an instructive series ; first, the 

 mesa tops, composed mainly of sandstone debris of an angular, 

 slightly worn character ; second, on the mesa-terrace the mantle 



1 See Pocket Map, Mon. 27, U. S. Geo]. Surv. 



