chap. xiy. Section of the Uspallata Range. 517 



formation is seen to be covered by (1st), a purple clay- 

 stone porphyry resting unconformably in some parts 

 on the solid slate, and in others on a thick fragmentary 

 mass : (2nd), a conformable stratum of compact blackish 

 rock, having a spheroidal structure, full of minute 

 acicular crystals of glassy feldspar, with red spots of 

 oxide of iron : (3rd), a great stratum of purplish-red 

 clay-stone porphyry, abounding with crystals of opaque 

 feldspar, and laminated with thin, parallel, often short, 

 layers, and likewise with great irregular patches of 

 white, earthy, semi-crystalline feldspar ; this rock 

 (which I noticed in other neighbouring places) per- 

 fectly resembles a curious variety described at Port 

 Desire, and occasionally occurs in the great porphyritic 

 conglomerate formation of Chile : (4th), a thin stratum 

 of greenish-white, indurated tuff, fusible and containing 

 broken crystals and particles of porphyries : (5th), a 

 grand mass, imperfectly columnar and divided into 

 three parallel and closely joined strata, of cream- 

 coloured claystone porphyry : (6th), a thick stratum of 

 lilac-coloured porphyry, which I could see was capped 

 by another bed of the cream-coloured variety : I was 

 unable to examine the still higher parts of the escarp- 

 ment. These conformably stratified porphyries, though 

 none are either vesicular or amygdaloidal, have evi- 

 dently flowed as submarine lavas : some of them are 

 separated from each other by seams of indurated tuff, 

 which, however, are quite insignificant in thickness 

 compared with the porphyries. This whole pile 

 resembles, but not very closely, some of the less 

 brecciated parts of the great porphyritic conglomerate 

 formation of Chile ; but it does riot probably belong to 

 the same age, as the porphyries here rest unconformably 



each other, and some miles southward they become actually united ; 

 this is a form of elevation which I have not elsewhere seen. 



