Page:Popular Science Monthly Volume 4.djvu/267

Rh terrible than that of the mouse or of a minnow swallowed by a perch, but there is a repulsiveness about the form, color, and attitudes of the octopus which invests it with a kind of tragic horror."

Cooling and Contraction of the Earth's Crust.—Prof. Dana concludes, in the September number of the American Journal of Science, a series of able papers on Dynamical Geology. He states that about 8 per cent, is the average change of density for the earth's crust between the stony and liquid states, which is equivalent to a change of volume from 100 to 92 per cent. This, therefore, expresses the contraction or shrinkage which the crust of the earth undergoes in its transition from a liquid condition to that of stone.

This contraction, as Prof. Dana long since stated, is the source of those inequalities of the surface which have resulted from a bending of the earth's solid exterior. From this cause have arisen the elevation of continents and the basin-like depressions now occupied by the waters of the oceans, and from the same cause mountain-chains have been uplifted.

The origin of the continental elevations and oceanic depressions was when the earth's crust began to form on the fiery liquid mass. Then, from change of density, already noticed, the cooled areas would sink and be overflowed by liquid matter, which, in its turn, would cool. Thus at length a solid and comparatively stable area would result—not elevated as yet, but at the general level of the liquid areas. These would, in their turn, undergo like change of conditions, and a crust, more or less stable, would envelop the globe. This would thicken, by solidifying, underneath the outer shell, as cooling proceeded.

But in thus solidifying it would undergo a change, both of density and volume, and this change would stand for a certain amount of contraction and subsidence. This amount, by the ratio given, would be in depression to an extent of 5,000 feet, if the crust or rocky layers be 12 miles thick. But the ocean-beds will average in depth, below the mean level of the continents, 16,000 feet. In order to effect so great a subsidence, the stony layers must be 38$1/4$ miles in thickness. In the subsidence several subordinate dynamical results must occur. One of these is powerful lateral pressure or thrust of the subsiding mass against the more stable areas, and this thrust might be horizontal, or obliquely upward. A consequence of this pressure would be an elevation or yielding, in some form, of the areas against which the pressure was directed. Possibly both have occurred; certainly the solid crust has bent, until vast mountain uplifts have occurred, and it became fractured to its profound depths.

From this and other considerations it would appear that continental elevations and oceanic depressions were outlined when the crust began to form, and that they have not since entirely changed places.

It further appears that the continents are a growth, in which additions to their margins have occurred. Such is evidently the case with the continent of North America, as shown in its rocks, in its outlines, and the character and results of its oscillations.

Improved Deep-Sea Sounding Apparatus.—In the July number of the may be found a description of Brooke's self-detaching shot-apparatus for bringing up specimens of the sea-bottom. This instrument has been considerably improved by Commander Belknap, of the U. S. steamer Tuscarora, now engaged in exploring the bed of the Pacific, with a view to find a suitable berth for a submarine cable from San Francisco to Japan, via the Aleutian Island chain. Commander Belknap's improvement consists, according to the Engineering and Mining Journal, of two cylinders, fixed one above the other when the instrument is set and descending through the water, and closing telescopically when the shot is detached on reaching the bottom. The lower cylinder is fitted with a conical cup at the lower extremity for the reception of parts of the bottom through an aperture, which, while descending, admits a flow of water upward through the cylinders by means of valves which close hermetically by the pressure of the water when the apparatus is being hauled up. The upper cylinder covers the aperture in the lower one on detaching the shot, so that the water