188 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The face of a great hill composed entirely of these rounded boulders 

 is seen on the west side of the track at , Mamei. The first bluff south 

 of Mata Chin is also composed of this rounded material. At Juan 

 Grande the face of the hill shows 110 feet of the formation. Another 

 is found about 100 feet south of Bas Obispo, while the Pena Xegra hill 

 previously described at the contact of the foraminiferal marls of the 

 Caribbean section is also composed of the material. 



Figure 7. Bluff at Mata Chin, showing Superposition of the Boulder Formation 

 upon massive Basic Igneous Rocks. 



At first sight of the exposure of these hills I was under an impression 

 that they represented the rounded exfoliated weathering so common in 

 doleritic rocks, but upon close examination I became convinced that 

 they were individual stones which had been rolled and accumulated, aud 

 are the debris of the older massive rocks. At several points the boul- 

 ders were found in unconformable contact with the underlying floor of 

 a basaltic rock, especially in the bluffs at Mata Chin and Bas Obispo. 

 They undoubtedly represent water or gravity rolled detritus of a time 

 when the igneous masses of the interior were much higher than now. 

 From the great thickness of this accumulation, and their occurrence 

 beneath the marine Tertiary rocks in at least one locality, Pena Blauca, 

 together with the fact that at both Juan Grande and Mamei the boul- 

 ders are overlain by old looking beds of finer and more consolidated 

 material resembling the tuffs and pumices of Barbacoas rocks, I am of 

 the impression that much of this debris is older than the Vamos a Vamos 

 Tertiary sediments of the Caribbean section. 



The outcrops of the boulder hills and bluffs, so far as my observation 

 extended, may be said to reach from Bujio to Bas Obispo, entirely on 

 the Caribbean side of the Isthmus. I could find nothing south of Cas- 

 cadas, at the north end of the Culebra subsection, or on the Pacific 

 slope, corresponding to this great accumulation of igneous debris on the 

 Caribbean side of the Isthmus. Beyond Cascadas to Paraiso the hills 



clay has been used for great deposits of igneous material embedded in red clay. 

 In such deposits the boulders are usually angular or little rounded. The hills we 

 are trying to describe, on the other hand, are made up of rounded, much rolled 

 pieces. 



