252 FINLAY 



a maximum size of six inches. Under the action of gravity, 

 and the occasional urging of torrential rains they are slowly 

 travehng down to the low ground in the valley bottoms. The 

 conditions by which weathering goes on in the San Jose region 

 are those in which a prominent part is played by the daily tem- 

 perature changes. Even in winter the extreme of cold is sel- 

 dom much below the freezing point, but the air is dry and the 

 nights are invariably cool. The maximum diurnal temperature 

 change in the exposed rock masses is not less than 90° F. 

 There can be no doubt that in the San Jose district that form 

 of weathering which is accomplished by the alternate expansion 

 and contraction of rock masses by heat and cold is of unusual 

 importance. The riving action of frost is here reduced to a 

 minimum. In the Baril Mountain range many great scales of 

 the nephelite syenite have been stripped away from the exposed 

 masses. 



The drainage of the San Jose valley is toward the north by a 

 stream which runs along the eastern flank of Mt. Anacuas, and 

 to the east by a trunk stream which leaves the valley under Mt. 

 Ladinas. The divide between the two systems coincides very 

 nearly with a line drawn from Mt. Armadillos to Mt. Baril. 

 About a quarter of the whole valley is drained by the second 

 stream. Many of the lesser divides are mere knife edges and 

 the Baril Range which lies between the drainage of the San Jose 

 valley and those streams which flow into the north fork of the 

 Arroyo Grande is so narrow that any further cutting must lower 

 its crest line. At no point, however, except near Mt. Ladinas 

 and Mt. Anacuas, have the arroyos which are everywhere gnaw- 

 ing headward against the ridge surrounding the valley of San 

 Jose succeeded in tapping its drainage. 



The Arroyo Grande rises against the Pic de Diablo six miles 

 to the south of San Jose and takes an easterly course. (See PI. 

 XVII.) The amphitheater which it drains, with the open valley 

 before it, bears a curious superficial resemblance to the cirques 

 and tributary valleys in a country which has been the home of 

 vigorous local glaciers. 



The Bocca de Alemos Arroyo rises beyond the Pic de Diablo 



