202 W. M. Gabb—Notes on Costa Rica Geology. 
rial, to say nothing of the shales probably derived from the 
same source. The absence of granite pebbles proves conclu- 
sively that this rock was not exposed at the time of the depo- 
sition of the Miocene. Another peculiarity of the conglomerate 
is that, while from the region of the Tiliri west and north, as 
far as the line of the railroad now being constructed, the peb- 
bles are hard and well rounded by attrition, and cemented by 
ori : 
bedded in a matrix formed from their own material, disinte- 
grated, and the whole, after deposition, to have undergone 
metamorphism together for the first time. . 
The trend of the granitic intrusion is not coincident nor co- 
extensive with the mountain range. It bears more to the east, 
Pico Blanco being its culminating point; beyond this to the 
east it extends almost to the Tilorio River, ending in a narrow 
tongue. I was unable to ascend the Tilorio, back to the granite 
belt; but although there are large streams penetrating west- 
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ailed discussion of this 
question here. I was three hours on the summit of the moun 
tain. It isa simple, straight ridge, of which the western end 
rises in a sharp cone, perhaps fifty feet higher than the rest 
There is no sign whatever of a crater, and 300 feet below the 
summit there is no volcanic rock. It must, therefore, be struck 
from the list of volcanoes. ; 
The two neighboring peaks, Mt. Lyon, between the Lar! and 
its branch, the Dipari, and U-jum, at the head of the Coen, are 
said by the Indians to be voleanoes. But after my experience 
with Blanco, I feel great hesitation in accepting the statement, 
although it is fortified by many very plausible stories of fires 
tip of what is otherwise an ordinar nite mountain. Space 
‘a det 
e 
and smoke seen on theirsummits. The latter, especially, ie 
a 
its sharp, regular cone, certainly- looks volcanic, and I 
hoped to reach it; but after my excursion to Pico Blanco, all 
hands of my party, white as well as Indians, decided that for 
the time, at least, it was impracticable. It would have cost 4 
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