PHILOSOPHICAL SOCIETY OF WASHINGTON. 55 



(abstkact.) 



I had the pleasure to accompany, as geologist, the party sent 

 out last year in command of Lieut. Geo. M. Wheeler by the U S. 

 Engineer Department to survey and describe certain little known 

 portions of Arizona and southern Nevada. The chief work was 

 geographical, and the corps of topographers was so large that 

 they were enabled to separate in detachments, connecting their 

 work at stated points. In this manner original geographical data, 

 adequate to the majiping of the mountain ranges, were obtained 

 over an area of 83,000 square miles. 



While there is great diversity in the character of the country, 

 a division is possible into two contrasted types — the cordillera 

 and the plateau, the disturbed and the undisturbed, the ridged 

 and the furrowed. 



The surface of Nevada belongs entirely to the first of these 

 types. From the Wahsatch range to the Sierra Nevada the face 

 of the land is corrugated in a system of ridges, few of them long 

 or lofty, but so frequent that one who should cross the country 

 in a right line would meet one, on the average, in every twenty 

 miles. These ridges coincide approximately with meridians and 

 are as near parallel as are the Wahsatch i-ange and the Sierra 

 Nevada. They are composed of (1) strata from Silurian to Trias, 

 more or less altered and uplifted, resting against (2) irruptive 

 granite and syenite. Over and about them are great accumula- 

 tions of volcanic rocks, but of these I will only say at present 

 that they are omnipresent in the field of our exploration. Further 

 south, in southeastern California and the southwestern half of 

 Arizona, there is a prolongation of the same system, with a de- 

 flection of the prevailing trend from N. and S. to N. W. and S. E., 

 and a further change from slightly altered to highly metamorphic 

 strata. A peculiarity of the cordillera country, due to climate, 

 is, that little of it finds drainage to the ocean, but it is partitioned 

 into numerous independent valleys, each gradually filling with 

 debris from the mountains. 



The plateau region, on the contrary, is well drained. Its strata 

 are unbroken and either horizontal or lifted m gentle undulations, 

 Denudation has scored it with deep chasms, and these, with the 

 tabular character of its uplands, are its distinguishing features. 

 To this system belong northeastern Arizona and great arens in 

 New Mexico and Utah. The limit of this district is an impor- 

 tant geographical and geological line, and its determination for 

 350 miles in Arizona is one of our most valuable geographical 

 results. 



This section [referring to a diagram] shows the rocks cut by 

 the Colorado River from the foot of the Grand canon to the Vir- 

 gin range. The notable points it illustrates are the profound 

 displacement of the strata that accompanied the upheaval of that 

 range, and the evidence afiorded of two epochs of folding. The 



