181) 



plugs. lavas and tuft's. The h-nas are liasalt. trachyte and rhyolite. Obsi- 

 dian also occurs in hirye iiuantities on the .Temez Plateau. The sedimen- 

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

 covered with lava. They were laid down in tlie seas and lakes that sur- 

 rounded the islands whicli now t'orni tlie high mountains of t^an Dia and 

 .lemez. These deposits date hack well into the Carhoniferous. and con- 

 tinue almost without lircak to the recent times. 



UIVEKS. 



The rivers of the region are the Itio Puerco. the Jemez and the Kio 

 Grande. The Kio Puerco, as we liave seen, closes in on the west the 

 region discussed in this paper: the Jemez River and its tributaries drain 

 the south and also the southwest slope's of the Jemez Mountains; and the 

 Rio Grande passes south through the section east of the Jemez Mountains, 

 and west of the San Dias. The Rio Puerco and the Jemez rivers are 

 tributaries of the Rio (Jrande. 



MOUNTAINS. 



The mountains, as h.is lu-en stated, are tlie Sau Dia and Jemez. The 

 former was caused by a fault of 11, (KX) feet along their western side, 7,000 



Little Pi^my \'cjU-;iiii 



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

 cap Carboniferous. The latter (the Jemez ^Mountains) h;ive a core of red 

 granite, overlaid in most cases, with hundreds of feet of volcanic delu'is, 

 except along the west wing of the group where tlie crest is granite. 



