122 zviR. J. eoma:nes q-n the [Feb. 19 12, 



feature of the junction is, that although the loose sand is baked and 

 bleached to a depth of 18 inches, the sand itself is quite undisturbed 

 by the overlying lava. A slice of this basal portion of the lava 

 shows it to be almost entirely glassy, with a few small phenocrysts 

 and some small rock-fragments which were evidently incorporated 

 by the lava while still fluid. The glass which comprises the bulk 

 of the rock is of a peculiar and distinctive purple colour, contains 

 a considerable amount of indeterminate opaque dust, and shows 

 extraordinarily complex flow-structure. Enclosed in this glass are 

 small crystals of very fresh labradorite, often corroded and con- 

 taining glass inclusions. Ferromagnesian minerals are represented 

 by a few, very small, colourless crystals of enstatite, together with a 

 little brown biotite. The included fragments are either glass, or 

 consist of a dark andesitic rock with felspar-microliths. 



Pig. 4. — Section througli the Rio Grande gorge at Cehadilla 

 to the Acfuacate Hills. 



[a = Older volcanic rocks of the Aguacate Series; 

 <6=0ld river-gravels and sands; c = Eecent lavas.] 



Above this basal modification comes the main mass of the 

 lava, which generally shows well-marked columnar structure and 

 weathers brown where exposed to the atmosphere. The rockpresents 

 a very heterogeneous appearance, quite different from the ordinary 

 andesites of the country. 



The matrix is dark grey, with many small and glassy-looking 

 plagioclase-crystals. Scattered through the rock are dark masses 

 and streaks of a very brittle black glass, which impart to the rock a 

 slightly banded appearance : on a weathered surface very intricate 

 flow-structure is seen, although on a fresh surface not a trace of 

 this can be made out. On the whole, the macroscopic characters of 

 the rock are much more like those of a tuflT than those of a true 

 lava; but I have no doubt that this rock is really a lava, though 

 certainly a very highly brecciated one. ^^ 



