presence of silt and mud, with shells, fish bones, and other remnants of aquatic life.

Professor S. Haughton has made some interesting calculations of the time required to remove one foot of earth from the whole of the rain basins of certain rivers, and finds the mean to be about 3000 years. The follow

Carrying his speculations further on this basis, and allowing the formation of strata to have gone on at ten times the present rate, he arrives at the conclusion that the whole of geological time is represented by a minimum period of 200,000,000 years.

"Faults" are frequently enlarged by denudation to a very great extent, until a wide interval is left between the two edges of the broken stratum, which again is filled with a "dyke" of fresh material. In this manner vast quantities of a rock have been removed, leaving the two cliffs of the originally dislocated formation far apart, and enclosing a bed of sand, clay, gravel, or other accumulation in the gap-the result of water action in some one of its forms.

Not only on the surface, but deep in the earth are hills and valleys, though the former do not exist as elevations, nor the latter as depressions, their contours

being masked by overlying rocks or sands. Remove these latter, however, and the old lines of cliff and shore, highland and lowland, lake and river, may in some instances be clearly traced. In other cases the sites of ancient lakes, sea-beaches, volcanic craters, lava streams, etc., may be made out, with forests and swamps and every other disposition of land with which we are familiar in daily experience. The proportions of land and water, the courses of rivers, and the shapes of continents have altered vastly in geological time; but the leading physical features of a period are sometimes firmly impressed upon its rocks, in which we have a mirror, broken, it is true, but still capable of reflecting, in its shattered fragments, many a landscape on which once shone the light of day.

Short as it is, the historical period affords abundant examples of both slow and sudden changes of level. In the year 1819 a remarkable depression took place in Cutch, in Hindustan, extending over 2000 square miles, converting this large area into a lagoon during the rains; and in the same year great activity was noticed in the Central American volcanic range, in which the crater of Jorullo occupies a conspicuous place. Even so recently as 1845 a minor subsidence occurred. The group of islands in the Greek Archipelago, which includes Santorin and Theresia, has received several additions since 186 B.C., and islands have appeared among the Aleutian group, the Philippines, Moluccas, Azores, etc., within the past century, and have become permanent additions to the land surface. In 1855 the ground of the

town of Wellington, New Zealand, rose two feet, and other places in the island much higher. The Chilian earthquake of 1822 elevated the shore-line several feet for a distance of a hundred miles, and emptied water-courses inland. The result, however, is not always permanent, for islands have frequently disappeared, and raised surfaces returned to their original level within a short time. The bed of the ocean is also the scene of disturbances of a volcanic character, similar to those which periodically affect the land. During the last twelve years the western coast of South America has been devastated several times by violent incursions of the sea, erroneously termed "tidal" waves, though they have no relation whatever to tidal action. The sea suddenly rises, and a wave or wall of water, twenty or thirty feet high, sweeps along the coast with irresistible force, swamping ships at their anchors or driving them ashore, and wrecking every building within reach of its influence. This is sometimes preceded by an equally sudden fall of the water, followed immediately by the "earthquake wave," as it undoubtedly is, with widespread destruction and loss of life. During a similar influx of the sea on the Island of St. Thomas, some ten years ago, a vessel of the United States navy was torn from her anchor, carried a long way inland, and left high and dry, with palm trees growing between her and the sea.

Sudden depressions and elevations of a considerable submarine area would have the effect of communicating a strong wave-like motion to the sea in the vicinity, and the frequently observed undulations of the land itself

during earthquakes confirm the belief that these catastrophes are due to the impulse so communicated. In some instances, earthquake waves arising at the foot of the Andes have apparently been propagated to islands far out in the Pacific, indicating the wide range of the propulsive force.

The network of telegraphic wires extending all over the world does good service in enabling accurate comparisons to be made of the times of occurrence of disturbances, and the sympathy between far distant volcanic regions has been ascertained by repeated observations. Some very interesting evidence has been collected by deep-sea exploring expeditions on the volcanic nature of the deposits on the floor of the ocean. Lava, volcanic ashes, etc., have been dredged up far from any active volcano-the most probable inference in these cases being that they have been ejected from submarine craters. Pumice-stone has also been met with by ships in almost incredible quantities, covering the sea for miles so thickly that the vessel has had difficulty in making way through it. In examining one of the charts of the sea bottom constructed by the scientific staff of the Challenger, one cannot fail to be impressed with the similarity to that of a mountainous land surface. Here are ridges of great elevation, sometimes serrated, sometimes rounded; valleys of great depth, abrupt precipices, gentle slopes, and circular crests of rock, sloping very gradually in some directions and going sheer down in others, and suggesting the crater form. The main lines of configuration are thus shown to be so similar to those prevailing

on land, that we cannot hesitate to ascribe to them the same origin. In confirmation of this, the statement of the captain of a British ship, as recorded in his log on January 29, 1878, is important, though it is only one of many similar observations. When in lat. 4° 20′ N., long. 21° 45′ W., several submarine volcanoes were seen throwing large columns of water a hundred feet into the air, the sea being in a state of great commotion, and a sound like distant thunder being audible. Two days before this date a destructive earthquake wave had visited Callao and other towns on the South American coast.

In the present state of our knowledge, or rather ignorance, little is to be gained by discussion on the probable cause of volcanic action. What sets up this action? It may be the sudden cracking of rocks under immense strain from subsidence-as when ice on a lake, the water level of which is falling, cracks in settling down, with a loud report and a movement quite perceptible to any one standing still upon it. The view at one time entertained, viz., that masses of molten matter beneath the surface were somehow set in motion, surging like waves against the more solid upper crust, is not so generally held as formerly. The generation of steam, and even its sudden condensation, may possibly be among the causes of the various exhibitions of volcanic energy which cannot be satisfactorily explained in the absence of any direct evidence. It is manifest that quantities of water do reach the underlying strata through fissures in the surface rocks, and become con