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 debris 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 debris. 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 outcrop more boldly, being exposed to the 

 summits of the high hills like those of Culebra, Lirio, and Paraiso. 



Figure 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 



