250 FJNLAY 



forks of the Arroyo Grande. The north fork of this stream 

 heads up under Mt. Baril and the Baril Range forms the divide 

 on the north between it and the San Jose valley. The south 

 fork of the Arroyo Grande has its source under the divide 

 formed by the continuation southwestwardly of the Baril Range. 

 The bottom of the valley of the Arroyo Grande is nearly three 

 thousand feet lower than the Pic de Diablo. By the profound 

 erosion which has taken place in this mountain valley the divide 

 between the streams flowing eastwardly from the San Carlos 

 Range and those which flow westwardly has been pushed far to 

 the west. At the head of the Bocca de Alemos Arroyo, which is 

 the stream in the next great valley to the south, the main divide 

 has been driven well over toward the eastern side of the range. 

 (See PL XVIII.) The valley of the Bocca de Alemos Arroyo 

 opens from a rock-walled canyon, as one proceeds up stream into 

 a broad elliptical amphitheater covering about six square miles. 

 This great depression lies well in toward the center of the moun- 

 tain mass. From the Pic de Diablo the San Carlos Range grad- 

 ually declines toward the southeast. It extends in this direction 

 for nine or ten miles from the Bocca de Alemos, and through- 

 out its course exhibits the characteristic mountain forms which 

 are found in the long lines of precipitous crags and sharp trail- 

 ing ridges of the Pic de Diablo. The San Carlos Mountains, 

 with their foot-hills, are a unit on the great plains of north- 

 eastern Mexico. (See PL XVI.) The lesser mountains which 

 border on the central chain commonly show forms of high re- 

 lief When they have been carved out of sedimentary rocks 

 their sides are invariably steep. They are at times precipitous 

 along lines of faulting. Their tops are continued by three or 

 four shoulders as pyramidal forms. When composed of igne- 

 ous rock they are often nearly perfect cones. The lowest out- 

 lying spurs of the range run down to the plains in rounded 

 contours, their line of slope being that of a parabola. The 

 San Carlos mountain mass is separated from the Sierra Madre 

 Mountains by a great plain, extending far to the northeast and 

 to the southwest. The distance across it between the two 

 ranges is fifty miles. When one travels across this plain it is 



