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- 

 quently 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 debris 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 clearly 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 At-enas across the Aguacate range. 

 These boulder clays are the same 

 which Belt mentions in his "Jour- 

 neys through Nicaragua," and to 



i-ii ,. . i i l-i ■ • i Figure 22. Section on Side of Road 



which he attributed a glacial origin. 1 _. _ , " 



_, . °.„ „ . ascending Rio Grande, Costa Rica, 



Mr. Sjogren and myself frequently opposite the 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 

 debris. Mr. Sjogren stated he had searched in vain throughout the 

 whole region, from Lake Nicaragua southward, for any evidence of gla- 

 ciation such as striae, morainic deposits, etc., and that in nearly every 

 locality the evidence was clear that the clays were decomposed vol- 

 canic ejecta. 



The volcanic heights of Costa Rica are chiefly built up by successive 

 accumulations of debris from the volcanic vents which have so long been 

 seeking successively higher and higher outlets through their own debris. 

 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. 



