Page:Popular Science Monthly Volume 34.djvu/679

Rh 2,000° F., sufficient to melt silver—more than sufficient to melt many lavas—would have been reached at a depth of about four miles. It would now be necessary to descend for at least forty miles in order to arrive at this zone. During the ninety-six years of the century it has been changing its position in the earth's crust, more slowly as time went on, from the one level to the other. There is another consideration. In very early times, as shown by Prof. Darwin and Mr. Davison, the zone in the earth's crust at which lateral thrust ceases and tension begins must have been situated much nearer to the surface than at present. If now, at the end of the century, it is at the depth of five miles, it was at the end of the fourth year at a depth of only one mile. Then, a mass of rock, ten thousand feet below the surface, would be nearly a mile deep in the zone of tension. Possibly this may explain the mineral banding of much of our older granitoid rock, already mentioned, and the coincidence of foliation with what appears to be stratification in the later Archæan schists, as well as the certainly common coincidence of micro-foliation with bedding in the oldest indubitable sediments. Pressure, no doubt, has always been a most important factor in the metamorphism of rocks; but there is, I think, at present some danger in overestimating this, and representing a partial statement of truth as the whole truth. Geology, like many human beings, suffered from convulsions in its infancy; now, in its later years, I apprehend an attack of pressure on the brain. The first deposits on the solidified crust of the earth would obviously be igneous. As water condensed, denudation would begin, and stratified deposits, mechanical and chemical, become possible, in addition to detrital volcanic material. But at that time the crust itself, and even stratified deposits, would often be kept for a considerable period at a temperature similar to that afterward produced by the invasion of an intrusive mass. Thus, not only rocks of igneous origin (including volcanic ashes) would predominate in the lowest foundation-stones, but also secondary changes occur more readily, and even the sediments or precipitates should be greatly metamorphosed. Strains set up by a falling temperature would produce, in masses still plastic, banded structures, which, under the peculiar circumstances, might occur in rocks now coarsely crystalline. As time went on, true sediments would predominate over extravasated materials, and these would be less and less affected by chemical changes, and would more and more retain their original character. Thus, we should expect that as we retraced the earth's course through "the corridor of time," we should arrive at rocks which, though crystalline in structure, were evidently in great part sedimentary in origin, and should beyond them find rocks of more coarsely crystalline texture and more dubious character, which.