. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 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 pec
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 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 tho
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Keywords: ., bookauthorha, bookcentury1800, bookdecade1860, booksubjectzoology
