HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 223 
immense thickness of red clay in which are more or less sparsely em- 
bedded irregular boulders of basic igneous material This is the so 
called * boulder clay " (see Plates XVI. and XVII.) of Tropical America, 
and it is by far the most widely developed and conspicuous geologic 
feature of the region. Vertical cuts of a hundred feet or more were fre- 
guently exposed, but they all reveal the same monotonous red clay and 
scattered boulders, and there can be no doubt but that the entire forma- 
tion of this bench is composed of the older igneous débris which has 
rolled down from the former heights of the interior country. The 
streams cut down below the summits of this bench, but nowhere did 
I observe a massive exposure of the original igneous rocks west of the 
Aguacate range. These clays, at least along this portion of the western 
coast, are olearly older in age and underlie the mass of material compos- 
ing the greater heights consisting of 
different species of more recent vol- 
canic ejecta, which will be seen as 
we proceed eastward from San Mateo 
to Atenas across the Aguacate range. 
These boulder clays are the same 
which Belt mentions in his “Jour- 
neys through Nicaragua,” and to _ 
which he attributed a glacial origin." ——— ШЫМЫ — 
l. ascending Rio Grande, Costa Rica, 
Mr. Sjógren and myself frequently opposite tlie Garita (Gabb). 
discussed the nature of these clays 
as we encountered them at various intervals almost to the Atlantic sea- 
board, and we could not convert ourselves to the glacial explanation, 
or consider the clays as being other than ancient oxidized volcanic 
débris. Mr. Sjógren stated he had searched in vain throughout the 
whole region, from Lake Nicaragua southward, for any evidence of gla- 
ciation such as strim, morainic deposits, ete., and that in nearly every 
locality the evidence was clear that the RA were decomposed' vol- 
canic ejecta. 
The volcanic heights of Costa Rica are chiefly built up by successive 
accumulations of débris from the voleanie vents which have so long been 
seeking successively higher and higher outlets through their own débris. 
The ejected material, greatly aided by the tremendous rainfall of the 
region, has found its way by gravity down the steeper slopes. The 
boulders represent the survival of the hardest in this process, while 
1 See * The Naturalist in Nicaragua," by Thomas Belt, London, 1888, pp. 247, 
260, 291. 
