248 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



remain as such, and are only indicated by the quasi cleavage-planes of 

 the rock. 



/. Shows no segregated minerals whatever, and is as porous as a 

 sponge. The vesicles are small, averaging 1 millimetre in diameter. 



g. Paste micro-crystalline ; its structure much obscured by decomposi- 

 tion. Compact; no vesicles. Color mottled red-brown and black. 

 Small particles of olivine are distinguishable, although decomposed. 

 This is essentially the variety^ c, without vesicles and a changed color, 

 the result of higher oxidation of the magnetite. 



h. Same as g in paste. Color reddish-drab. Minute irregular cav- 

 ities, produced by decomposition of certain mineral constituents. 

 Olivine inclosures reaching a diameter of 2 millimetres. This is a still 

 further progressed product of decomposition. 



i. Very much like /. Color grayish-brown, thoroughly vesicular. 

 Not only are the small vesicles found as in/, but large ones occur, show- 

 ing a glazed surface. All of them have been more or less drawn out. 

 In its texture it closely resembles pumice. 



Station 104. Basalt. 



a. Paste micro-crystalline ; color black. Minute crystals of feldspar 

 and finely distributed olivine give the rock a glassy lustre. Innumer- 

 able small vesicles. Some larger ones are scattered throughout. The 

 latter are filled with either crystalline or amorphous carbonate of lime. 



Station 99. Basalt. 



a. Paste grayish-black when fresh, reddish-brown when decomposed ; 

 crystalline. Black biotite in minute crystals. Olivine brown. Minute 

 vesicles and scattering larger ones, both irregular. 



It appears that after the outpouring of this basaltic lava there was 

 an elevation of the region directly west involving a portion of the ba- 

 salt. Judging from the eastern limits of the area, it seems probable 

 that the rise westward did not exert its influence for any long distance 

 into San Luis Valley. 



As one of the characteristics of this region we may regard the "fresh" 

 appearance of the basaltic rocks exhibited at many places. Very little 

 decomposition and disintegration have taken place, thus preserving, in 

 a great measure, the surface intact. This naturally results in a lack 

 of vegetation, which, in turn, augments the appearance of freshness. 

 In this connection reference may be made to an important fact, which 

 is not unfreqnently overlooked. Owing to the fact that decomposition 

 has but slightly affected the rocks, only a scant growth of vegetation 

 can subsist. Prom this circumstance the inference is usually drawn 

 that the lava assumed rigidity but a comparatively short time ago. On 

 the other hand, a rich growth of plants is regarded as an argument for 

 greater age. It remains to be remembered, however, that if once grow- 

 ing vegetation can gain a hold in the soil, disintegration and decompo- 

 sition of the rock will progress at a far more rapid ratio than before. It 

 is the formation and retention of the first soil that requires a long period 

 of time, hence exposures that look very " fresh " are not necessarily of 

 relatively recent date. 



Besides the two mountains above mentioned, there are several other 

 prominent points that I regard as owing their existence to the fact of 

 being points of eruption. No definite arrangement can be made out in 

 their horizontal distribution. It may be said, however, that they occur 

 at such points or along such lines, where the underlying trachorheites 

 probably offered the least resistance to vertically-acting forces. 



As the main characteristic of this very large area, we may regard the 

 surprising uniformity shown by the basalt, except at the points of ejec- 



