188 BULLETIN: MUSEUM OF GOMPARATIVE 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 blutf 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 Negra hill 
previously described at the contact of the foraminiferal marls of the 
Caribbean section is also composed of the material. 
Бісгвв 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, and 
are the débris 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 Blanca, 
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 débris is older than the Vamos 4 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 débris 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. 
