332 GEOLOGICAL SURVEY OF THE TERRITOEIES. 



Another small mass of basalt occurs in tlie Sangre de Ohristo range, 

 among the foot-hills, east of station 16, where several small hills of it 

 were ejected, somewhat disturbing the superincumbent sandstones. 

 Looking off to the east and southeast from station 84, a number of cones 

 were noticed among the Tertiary formation, that are probably basalt, 

 but no examination of the locality could be made. Altogether, this 

 occurrence of basalt is very limited, compared with that of other dis- 

 tricts ; but it seems probable that the presence of such large quantities 

 of trachorheites might account for it. 



Dikes occur in large number and considerable extent in section b. 

 By far the most numerous are the hornblende-dikes, with a usual strike 

 of north to south and almost vertical dip. While they are not quite so 

 frequently met with in the northern portion of the granitic mass, they 

 abound in the southern, sometimes traceable for several miles, but 

 almost invariably of limited lateral extent. tJrdinarily they carry a 

 considerable amount of quartz and hornblende, mostly mica, and in 

 some instances feldspar, garnets, and epidote ; the two latter either as 

 crystals or massive. Their texture is frequently that of gneiss, owing 

 to the hornblende, to the mica, or both, whereby they are distinctly offset 

 from the surrounding granite. When variations of texture take place, 

 these rocks resemble a schist more closely, aud may give rise to misinter- 

 pretations. In the Sangre de Christo range we find a large number of 

 such dikes, with the usual strike passing through the sandstones, and 

 farther north through the granite that is exposed at the base of the 

 range, but never, as far as I have observed, through the eruptive 

 granite. Differing from the eastern ones, these retain their mineralogi- 

 cal and lithological character more uniformly throughout, and resemble 

 diorite. In some instances, when the quantity of mica accumulates to 

 a considerable extent, the texture becomes more like that of a schist, the 

 structure more stratified, and the hornblende not rarely is segregated in 

 single prismatic crystals of about an inch in length. It is possible that 

 the hornblende-schists, occurring at the northern end of the range in 

 question, should be regarded merely as immense dikes of this character, 

 as in their mineralogical composition there is no essential difference. 

 The presence of feldspar in the dikes of the range, compared with the 

 scarcity in the eastern ones, is a point worthy of notice, and it does not 

 occur in the same manner again to that extent in any of the dikes 

 throughout the district. What the geological relations of these dikes 

 are seem a little obscure, but they may perhaps justly be regarded 

 as segregations in the greater number of instances. 



Section h includes a large drift-area. Along the south side of the 

 Arkansas, a narrow strip of drift-deposit occurs, the deposition of which 

 was facilitated by a widening of the canon. Wet Mountain Valley is 

 covered entirely by drift, originating from the mountains on either side 

 of it. Drainage has cut a number of steep gorges into the loose mate- 

 rial, of no very considerable depth, however. The grandest mass of 

 drift-material, or, perhaps more correctly speaking of fluvial deposit, is 

 found in San Luis Valley. Over thirty miles long, to the southern litnit 

 of section ft, and at that point nearly thirty-five miles across, stretches 

 an uninterrupted plain, the soil of which is made up entirely of drifted 

 material. On the western side of the valley, the bluff's of gravel rise to 

 probably more than 100 feet above the ordinary level; but eastward 

 nothing of the kind is observed. Toward the center of the valley its 

 greatest depression occurs, and there San Luis Creek flows southward, 

 emptying into the small San Luis Lakes. A large portion of the 

 ground is swampy and impassable. At the eastern border of the valley 

 the gravel bluffs are not so prominent as on the western, but a very 



