CHAP. V A GEOLOGICAL SKETCH 135 
1 accumulated, bearing on this point, it may assist 
abler physicists than myself to decide the question. 
I have spoken before of a conical peak, called the 
Serra d' Irura, looking from afar like enough to a 
small volcano. It lies S. 37° W. and four miles away 
from Santarem, and I suppose it may rise to 300 
feet above the Tapajoz. This peak is overstrewn 
with scoriae of the kind described, but the top is 
rounded and there is no semblance of a crater. 
Beyond it are ridges, strewn with similar blocks, 
and with intervening hollows ; but the latter are 
not crater-like, and there is no trachyte or basalt 
or volcanic rock of any kind beside those boulder- 
like blocks ; nor, although there has been much 
denudation, as we shall presently see, is there any 
rernarkable tilting up of such stratified rocks as are 
still in situ. 
I may here enumerate all the sites in the Amazon 
valley where I have seen these volcanic boulders, 
beginning at the coast and proceeding westward. 
The first is Caripi, near Para, in lat. about S., 
long. 48I-" W. ; (2) Santarem, long. 54^40' W.; (3) the 
cataracts of the Aripecurii, lat. 0° 47^ S., long. 56" W. ; 
(4) Villa Nova, long. 57° W. ; (5) on the Parana- 
miri dos Ramos, long. 57° W. ; (6) Serpa, long. 58° W. ; 
(7) on the Rio Negro, at various points for a short 
distance up, long. 60° to 60^° W. ; (8) Manaquiry, on 
the Upper Amazon, lat. about 4° S., long. 6o|^° W. 
This is the most westerly point at which I have 
observed them myself, although I have been told 
of their existence at several intermediate points, 
and also at Coari, still farther west. It may well 
be supposed that when I reached the foot of the 
Andes I looked to find them much more abundantly ; 
