Page:Popular Science Monthly Volume 52.djvu/273

Rh elevation of which one would ask one hundred thousand years, the other might require for its more sudden elevation a force which had taken the same number of years to accumulate its energies.

Discussing in the Geological Society in 1889 the distribution and probable age of some palæolithic flint implements, Professor Prestwich maintained that the removal of the material observed indicated the existence of agents of greater force than those operating under the present river régime. This closed up the time required for the completion of the great physical phenomena, though the author's inquiry tended to carry us further back geologically than was usually admitted.

In a paper read at the British Association in 1881 on the causes of volcanic action, Professor Prestwich presented objections to the generally accepted theory of Scrope that eruptions are a phenomenon of steam, and held that water, instead of being a primary was only a secondary cause of them—the primary cause being the rolling up of the lava in consequence of pressure due to a slight contraction of the earth's crust. The contact of this fluid lava with the water stored in the crevices is followed by a flush of steam, and this by an influx of water from underlying strata. These are converted into steam and expelled, and the exhausted strata serve as a channel for the influx of sea water into the volcano. A point is finally reached when by the cessation of the shocks and excessive drainage the flow of lava is effected quietly. A paper on Regional Metamorphism, read to the Royal Society in 1885, presented a theory that there exists, in the compression and motion of the strata which have always accompanied the upheaval of mountain chains, a true cause for the development of an amount of heat sufficient to produce one form of metamorphosis—a form which can affect only particular regions—and he would, therefore, in order to distinguish it from contact and normal metamorphism, designate it as regional metamorphism.

Some of Professor Prestwich's later views respecting the Glacial period were presented at a meeting of the Geological Society in May, 1887, when, after showing how the discoveries in the valley of the Somme and elsewhere, twenty-eight years before, had led geologists who had previously been disposed to restrict the age of man to exaggerate the period during which the human race had existed, he proceeded to discuss the views of Dr. Croll on the date of the Glacial period. In view of the recent observations in Greenland of Professor Helland, Mr. Steenstrup, and Dr. Rink, showing that the movement of ice in large quantities was much more rapid, and consequently the denudation produced was much greater than had formerly been supposed, he was disposed to limit the duration of the Glacial epoch to from fifteen to twenty thousand years,