CHAPTEH IV. 



GEOLOGICAL OBSEEVATIONS ON THE COUNTRY ALONG THE BOUNDARY LINE 

 LYING BETWEEN THE lllTH DEGREE OF LONGITUDE AND THE INITIAL 

 POINT ON THE RIO COLORADO. 



[By Arlhur Scliott, Assistant U. S. B. C] 



Geographical terms for this section of the boundary line would be Sonorian or Pimerian, as 

 it runs through the northwestern part of Sonora, which also bears the old Spanish name of 

 Pimeria Alta (High Pimeria ;) and since it intersects both meridians and parallels in an oblique 

 direction, it is called, in geodetic language, "azimuth line." This line lies entirely on the 

 eastern slope of the basin of the Gulf of California, and falls on the divide separating the waters 

 of the Gila from the streams of Northern Sonora, which, after flowing in a southwesterly course, 

 empty into the Gulf of California. 



The hypsometrical and general geological features can only be expressed approximately, for 

 circumstances prevented actual measurements. 



At the eastern end of the azimuth line is the Sierra del Pajarito, from the highest point of 

 which an imaginary line drawn to the Rio Colorado would give a grade of about 22 feet to one 

 mile, or its equivalent, 0.41 to 100. The highest point may be set down at about 5,200 feet 

 above the level of the sea. This point does not, however, reach the pine region, which in this 

 latitude may be considered as occurring at an elevation of not less than 6,000 feet. A monoto- 

 nous simplicity is a characteristic of the topographical features of Northwestern Sonora ; and 

 but for a close examination, there would only be disclosed a mere dualism of diluvial drift and 

 pluto-volcanic mountains. The drift covers many of the mountain ranges almost to the tops, 

 particularly those which approach the bottom-lands of the Colorado. 



The northwestern part of the line runs over what may be called a veiled country ; for of the 

 mountains, only their crests are to be seen above the desolate sand-flats of the general level of 

 the surface. It is through these forsaken barrens that the Rio Colorado, with its timbered 

 bottom, winds its course towards the waters of the Gulf. 



Comparing the geological edifice with the structure of animal organism, the mountain ranges 

 jutting up through this vast level of drift represent the skeleton ; the diluvial main the sinew 

 and mu.scle ; and the alluvial deposits the tegument or epidermis. The last mentioned is poorly 

 represented. The scant vegetable cover facilitates, however, the observations of the geologist. 



Alluvium is seen first and as the uppermost stratum ; except at the extremities of the line, 

 there is, however, but very little to be met with. As might be expected, it abounds most in the 

 bottom-lands of the Colorado ; but, strange to say, it is even in greater abundance on the- highest 

 movmtains than on the plains. It frequently collects in such quantity in the little valleys and 

 in the cavities of the broken sides of the mountains as to give rise to a more complete develop- 



