HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 189 
are almost entirely composed of the massive igneous rocks or consoli- 
dated tuffs. 
'The disappearance of these boulder hills to the southward, coincident 
with the point where our section leaves the Chagres, may suggest that 
they represent accumulations in the ancient valley of that stream. 
But the absence of intermixed river silt or pebbles of different rocks 
stands in opposition to such an hypothesis, especially when taken in 
connection with the fact that these peculiar boulders are not found in 
any of the present river deposits. I am inclined to think that they 
represent rolled hillside volcanic débris of contemporaneous age with 
the great basic igneous eruptions of Cretaceous and Eocene time. 
The Massive Igneous Rocks. — Near the 25-mile post from Colon, be- 
tween Baila Monos and Mamei, beneath a high bluff of the Mata Chin 
boulders, the first exposure of the massive igneous rocks is encountered 
in the cuts of the railway and canal, and from thence on to Paraiso they 
are frequently seen. Specimens were collected from every possible out- 
crop, the petrographic nature of which has been kindly determined by 
Professor Wolff. These massives are all ancient, half decomposed dark 
colored rocks, 
They do not now occur as horizontal sheets or lava caps, but project 
vertically from below as if the remnants of ancient protrusions, the 
upward continuation of which has long since been destroyed or usu- 
ally buried by later débris. Owing to the great subaerial decay and 
dense vegetation it was utterly impossible in this region to trace these 
rocks to the summits of the hills. Between Mamei and Cascadas they 
are only exposed in the fresh cuts of the canal and railroad in the base 
of the innumerable high hills which characterize this region. From 
Cascadas to the Pacific they outerop more boldly, being exposed to the 
summits of the high hills like those of Culebra, Lirio, and Paraiso. 
Fieurn 8. Section near Mamei, showing Gradation of Mata Chin Boulders 
into finer Material. 
The accompanying figure of the outcrop at Mamei and the illustration 
of the canal cutting at San Pablo (Plate X.) show how these rocks are at 
present exposed. Outcrops of this nature were studied at Mamei, Mata 
