39 2 Tertiary Formations. part n. 



lavas. This pitclistone, as well as some purple clay- 

 stone porphyry, certainly flowed in the form of streams. 

 These various lavas often pass, at a considerable depth 

 from the surface, in the most abrupt and singular 

 manner into wacke. Great masses of the solid rock are 

 brecciated, and it was generally impossible to discover 

 whether the recementing process had been an igneous 

 or aqueous action. 1 The beds are obscurely separated 

 from each other ; they are sometimes parted by seams 

 of tuff and layers of pebbles. In one place they rested 

 on, and in another place were capped by, tuffs and grit- 

 stones, apparently of submarine origin. 



The neighbouring peninsula of Lacuy is almost 

 wholly formed of tufaceous deposits, connected probably 

 in their origin with the volcanic hills just described. 

 The tuffs are pale-coloured, alternating with laminated 

 mudstones and sandstones (all easily fusible) and passing 

 sometimes into fine grained white beds strikingly re- 

 sembling the great upper infusorial deposit of Patagonia, 

 and sometimes into brecciolas with pieces of pumice in 

 the last stage of decay ; these again pass into ordinary 

 coarse breccias and conglomerates of hard rocks. With- 

 in very short distances, some of the finer tuffs often 

 passed into each other in a peculiar manner, namely, 

 by irregular polygonal concretions of one variety in- 

 creasing so much and so suddenly in size, that the 

 second varietv, instead of anv loup-er forming the entire 

 mass, was left merely in thin veins between the concre- 



1 In a cliff of the hardest fragmentary mass, I found several 

 tortuous, vertical veins, varying in thickness from a few tenths of an 

 inch to one inch and a half, of a substance which I have not seen 

 described. It is glossy, and of a brown colour ; it is thinly laminated, 

 with the lamina? transparent and elastic ; it is a little harder than 

 calcareous spar ; it is infusible under the blowpipe, sometimes decre- 

 pitates, gives out water, curls up, blackens and becomes magnetic. 

 Borax easily dissolves a considerable quantity of it, and gives a 

 glass tinged with green. I have no idea what its true nature is. 

 On first seeing it, I mistook it for lignite ! 



