TO JUNCTION OF GRAND AND GKEEN 1UVEKS. 101 



CHAPTER VI 



GEOLOGY OF THE BANKS OF THE SAX JUAX. 



General features of the country bordering the San Juan — Section of Lower Cre- 

 taceous STRATA SOUTH OF SlERRA ABAJO — BlRD'S-EYE VIEW OF COUNTRY BORDERING 



San Juan— High mesas of tiie Navajo country— Triassic rocks of Lower San 

 Juan— Lower Cretaceous strata of Camp 37— Middle Cretaceous beds and fos- 

 sils south of La Late— The Needles— The Creston— Upper Cretaceous strata 

 at mouth of the animas— ruins in the san jl'an valley— canon largo— sections 

 of Upper Cretaceous strata— Plateau country bordering Canon Largo— Buttes 

 of marls and sandstones, highest members of the series of sedimentary rocks 

 composing Colorado Plateau — General section of Upper Cretaceous strata— 

 Nacimiento mountain, its structure and relations — Notes on the different for- 

 mations EXPOSED ON ITS SIDES — JOURNEY THROUGH COUNTRY BORDERING WESTERN BASE 



of Nacimiento Mountain— Divide between San Juan and Rio Grande— Mount Tay- 

 lor— Cabezon — Tertiary strata at Jemez— The Valles— Trap Plateaus. 



The great features in the structure of the country traversed bv the San Juan 

 River have been given in a preceding chapter, and there now remains for the comple- 

 tion of the picture only the presentation of the more local details upon which the gen- 

 eralizations before advanced were based. 



Anticipating in some degree the progress of our geological narrative, to render 

 the subject more readily comprehensible, the following outline sketch of the geology 

 of the immediate banks of the San Juan is here given : 



Taking their rise in the valleys dividing the parallel ranges of the Rocky Moun- 

 tains, Sierra de la Plata, Sierra de los Pinos, Sierra San Juan, &c, the tributaries of the 

 San Juan unite in the open country lying south of these mountain ranges to form a chan- 

 nel of drainage, having nearly an east and west course, to the Colorado. In addition 

 to the tributaries of the San Juan which have been enumerated, far less important ones 

 flow from the east and south. These are the rivulet traversing Canon Largo, rising at 

 the north end of the Nacimiento Mountain, the Ohaco, Gothic Creek, and the Chellv, 

 which drain the arid mesas of the Navajo country and the slopes of the Tunecha and 

 Carrizo. 



The country traversed by the northern and more important branches of the San 

 Juan, bordering our outward-bound trail, has been fully described in the preceding 

 chapters. That bordering Canon Largo and the course of the Chaco opens to us a new 

 geological field, and one which includes the exposures of the highest portion of the 



v x 



