'aughan.] STRUCTURE OF SAN CARLOS FIELD. 75 
m its western side, siibparallel to it, is a lower range of unnamed 
nountains whose altitude rarely exceeds 4,250 feet. 
For a considerable portion of its length the rocks on the east side 
>f this stream dip toward the Vieja Mountains, and on the west side 
,oward the range on the west. Therefore this creek frequently flows 
tlong the very summit of an anticline, of which the Sierra Vieja rep- 
resents the eastern limb and the unnamed range to the west the west- 
ern limb. On the southern side of Chispa Summit the dip is south- 
erly. About 2 miles southwest of the summit, and subparallel to 
:he Rio Grande Northern Railroad, there is a narrow ridge with an 
iltitude of approximately 4,500 feet, or some 500 feet above the val- 
ey. This ridge is composed of Lower Cretaceous limestone. Along 
Its eastern front there is a fault, with a north-south strike and an 
easterly downthrow, by which the Benton shales have been brought 
iown about 200 feet below the top of the Edwards (Caprina) limestone, 
to that now the shales are exposed from the foot of the ridge to a line 
3ast of the railroad. 
San Carlos, with an altitude of about 3,988 feet, is situated in the 
basin valley of San Carlos arroyo and is almost entirely enclosed by 
mountains belonging to the Vieja system. The highest point of the 
Vieja Mountains, about 7 miles east of the town, is 6,470 feet. (Pis. 
VII, VIII, and IX.) The summit of the mountain is a little more than 
2,700 feet above the valley of San Carlos arroyo. The basin is almost 
completely surrounded by what the inhabitants call rim rock — quartz- 
pantellerite — which occurs east, south, west, and north of the town. 
Around the border of this basin the rocks dip away from the center, 
those along the eastern side dipping toward the east, those on the 
southern side dipping toward the south, and those on the western and 
northern sides dipping west and north, respectively. The geologic 
structure is, therefore, that of a quaquaversal fold or an anticlinal 
dome. San Carlos arroyo has cut out the top of the dome and made 
its bed across the southwestern corner, flowing in a southerly direction 
into the Rio Grande. Just west of San Carlos there is a great fault, 
pointed out by Dumble, 1 striking north and south, with ** westerly 
downthrow of over 2,000 feet, and bringing the rim rock "nearly to 
the level of the valley." 
Along the foot of the mountains on the west side of San Carlos 
arroyo, and within a distance of three-fourths of a mile, are five springs, 
known as Newman's springs. The largest has been gaged, and accord- 
ing to Mr. S. A. Johnson furnishes a supply of 15,000 gallons of 
water a day. Mr. Johnson stated that he believed when all the springs 
are cleaned out that they will supply 40,000 to 50,000 gallons a day. 
The water is of excellent quality. 
i Bull. Geol. Soc. America, Vol. VI, p. 387. 
