Page:Popular Science Monthly Volume 16.djvu/892

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The Old River-Beds of Middle California.—Old river-beds are found in nearly all countries which have been affected by drift agencies. They are also found in California, but, while in other parts of that State they present general features similar to those of the Eastern States, those of the auriferous slate belt of middle California are entirely different in character and in their situation as respects the present river-beds, and are in some respects unique. In most other countries the present river-beds occupy the same position as the old; the rivers of middle California have been displaced by lava-flows from their former position, and compelled to cut entirely new channels. Instead of cutting these channels at a higher level than the older ones, as has been usually observed, the displaced rivers in California have cut them two or three thousand feet deep in the solid slate, leaving the old detritus-filled channels far up on the dividing ridges. In other parts of the United States the drainage system has remained substantially unchanged since Tertiary times, but in this California region the new drainage system is entirely independent of the old, having the same general direction, but sometimes cutting across it. The detritus in the old river-beds of California is composed of large pebbles and bowlders, instead of the silts generally found, and is capped by lava or other volcanic material. In these observations Professor Joseph Le Conte and Professor Whitney substantially agree. Professor Le Conte has made a further study of the phenomena, and has given his conclusions in a paper in the last number of the "American Journal of Science." The old stream-beds, as they arc exposed in the processes of hydraulic mining, are shallow, lowest in the middle and rising to the sides, with such forms ground upon the surface of the bed-rock at their bottom as are always produced by swift currents carrying coarse materials, and are in marked contrast with the deep, sharply Y-shaped cañons which characterize the present rivers in the same region. The filling up of the beds consists of a lower course, sometimes a few feet, sometimes many feet in thickness, of a conglomerate of pebbles and bowlders of; considerable size, cemented with sand and a blue clay. Above this are alternate layers of pebbles, gravel, sand, and clay, with fragments or trunks of trees of the Pliocene age, and bones of the mammalia of the Pliocene and Quaternary ages, and perhaps human relics. Above the detritus is a capping of volcanic matter, a tufaceous conglomerate, with or without basalt over it. Professor Le Conte accounts for the singular phenomena by a theory, the principal features of which are that the old drainage system began to be formed after the birth of the Sierra Nevada, at the close of the Cretaceous period, and continued to exist through the Tertiary; the Sierra rose during the Glacial period, accumulated great masses of snow and ice and glaciers that were to play a part in filling the beds, and made the courses of the rivers much steeper than they had been before. The eruptions which threw out the volcanic products were preceded by a period of underground heat which melted the accumulated ice-masses. The waters and ice rushing in violent torrents brought down the coarse gravels and masses of rock, and dropped them as they became too great a load for the streams to carry. Afterward came the eruptions, first of ashes, then of lava, which flooded the mountain-slopes and completely obliterated the drainage system. Coincidently there were a considerable elevation of the Sierra range and an increase of the mountain-slope. The glaciers and rivers now began to cut a new system of channels independent of the old ones. They preferred the old divides, for the lava was thinnest or wanting there. As a necessary consequence of the increased elevation, the new channels were cut down to a level below that of the old ones.


Age of the Green Mountains.—Professor James D. Dana gives, in the "American Journal of Science" for March, his reasons for having in the new edition of his "Geology" referred the epoch of the formation of the Green Mountains—in the system of which he includes the whole region between the Connecticut and the Hudson—to the close of the Lower Silurian period. They are, when summarized: 1. That the western half of the region is proved to consist of rocks of the Lower Silurian age, and of one