181) 



pluys. lavas ami tuHs. The lavas are liasalt, trachyte and rhyolite. Obsi- 

 dian also occurs in large quantities on the Jemez Plateau. The sedimen- 

 tary deposits are the country rocks of nearly the entire region where not 

 covered •with lava. They were laid down in the seas aiul lakes that sur- 

 rounded the islands which now form the high mountains of San Uia and 

 Jemez. These deposits date hack well into the Carlioniferous, and con- 

 tinue almost without lireak to the recent times. 



The I'ivers of The region are tlie Kio Tuert o. the Jemez and the Rio 

 Grande. Tln' Kit) I'uerco. as we have seen, closes in on the west the 

 region discussed in This paper: The Jenu^z Kiver and its triliuTaries drain 

 the south and also the southwest slopes of the Jemez Moiuitains; and the 

 Rio Gi'ande passes soiTth thi-ough the section east of the Jemez Mountaius. 

 and west of the San I Has. The Rio I'uerco and the Jenu-z rivei's are 

 tril)utaries of the Rio Grande. 



MOI'XTAIXS. 



The moTintains. as has lieen statt d. are the San Dia anil Jemez. The 

 former was caused by a fault of ll.(MH> feet along their western side. 7,0(K» 



Littlo Piginy V.il;-;uin. 



feet of which still remain, as an escarpment. Their core is granite, their 

 cap C'arlionifei-ous. Tlie latter (the Jenuz ^Mountains) have a core of red 

 granite, overlaid in most cases, with Imndreds of feet of volcanic debris, 

 except along the west win.g of the group where the crest is granite. 



