360 



AAIERICAN FORESTRY 



alone on the great plain, which toward 

 the east, south, and west stretches 

 away nearly level as far as the eye can 

 reach. It is the largest and probably 

 the oldest of the extinct volcanoes in 

 the region described. If there are 

 older volcanic mountains the evidence 

 of their extrusive origin has not been 

 iound. Its altitude is given on some 

 of the maps as 11,150 feet, but my 

 aneroid registered little more than 

 8,000 feet at the top. The cone is 

 nearly circular in outline and the slopes 

 of the sides are gentle. There is a 

 fairly well-marked crater at the sum- 

 mit, but one side is broken down, leav- 

 ing a crescent-shaped rim enclosing the 

 old crater which was estimated to be 

 half a mile across. This rim is gently 

 rounded at the top, and its breadth in 

 some places is quarter of a mile or 

 more. 



The rock is dark colored andesitic 

 lava varying in character from vesic- 

 ular to compact. No cinders or scori- 

 aceous material was found on the 

 outslopes, but beds of red cinders occur 

 within the crater. The slopes seem to 

 be made up of successive flows of lava 

 having approximately the same gradi- 

 ent as the mountain slopes, so that the 

 mass seems to be composed of con- 

 centric layers like an onion. The outer 

 edge appears to be lobed, due to the 

 extension of some flows beyond the 

 limit of others, but this character was 

 noted only from a distance. No evi- 

 dence of explosive action was found 

 in Sierra Grande. The character of 

 the rock and the gradient established 

 by its flows indicate a volcano of quiet 

 action in which the lava poured over 

 the rim or broke through the side with- 

 out violent demonstration as that from 

 Kilauea does at the present time. 



Canyons have been cut to a depth 

 of 200 feet or more in the sides of 

 Sierra Grande. Erosion to such a 

 depth in hard andesitic rock in a semi- 

 arid region where the only water avail- 

 able for erosive work is the slight 

 amount that falls on an isolated cone, is 

 evidence of a long period of time. No 

 canyons at all comparable to those of 

 Sierra Grande were found on other vol- 



canic cones of this region. This, to- 

 gether with the subdued form of the 

 cone and the rounded contours of its 

 surface, seems to place Sierra Grande 

 in a class apart from the other volcanic 

 mountains of the region and to prove 

 that it is the oldest of the cones now 

 known to be of extrusive origin. 



ROBINSON MOUNTAIN 



Robinson is the name given to a vol- 

 canic mountain located about 7 miles 

 southwest of Folsom, New Mexico. It 

 has an altitude of about 8,000 feet, but 

 inasmuch as the cone rests upon a 

 broad and rather high mesa it is much 

 less conspicuous than the neighboring 

 mountains of about the same altitude. 

 The sides of the cone are steep in some 

 places, but on the whole the approach 

 to the summit is easy. There is a well- 

 defined depression in the summit but 

 the confining rim is broken away on 

 one side so that the crater has a cirque- 

 like form. The rock is highly scoriaceous 

 and much of it has the character some- 

 times known as "rock foam," that is, 

 the gas cavities constitute so large a 

 proportion of the rock that it will float 

 in water like a cork. In some of this 

 rock the gas cavities are so uniform in 

 size and so regularly distributed that 

 some people who are ignorant of its 

 origin call it petrified honeycomb. 



The fact that Robinson Mountain 

 is younger than Sierra Grande and 

 older than Capulin is proved in several 

 ways. Although composed of rocks 

 much softer than that of Sierra 

 Grande, it has not been so deeply dis- 

 sected by erosion and it rests on a 

 lava platform much lower than the 

 lavas of the high mesas that resulted 

 from the earlier volcanic eruptions. On 

 the other hand, the platform is much 

 higher than that on which Capulin 

 stands, and the rounded outlines and 

 soil-covered surface are in marked con- 

 trast with the rough, angular outlines 

 and fresh appearance of the "mal pais" 

 surrounding the younger cones. 



CAPULIN MOUNTAIN 



Capulin is the name given to a mag- 

 nificent example of extinct volcano near 



