descends by a set of ledges or steps, their form ought to be particularly attended to; and whether they are covered with sand or by dead or living coral; and whether the corals differ on the different ledges: the same points should be attended to within the lagoon, wherever its bed or shore is step-formed: the origin of these steps or ledges is at present obscure. In the Indian and Pacific Oceans there are entire reefs, having the outline of atolls or lagoon-islands lying several fathoms submerged; there are likewise defined portions of reefs both in atolls and in encircling reefs similarly submerged. It would be particularly desirable to ascertain what is the nature of these submerged surfaces, whether formed of sand or rock or living or dead corals. In some cases two or more atolls are united by a linear reef; the form of the bottom on each side of this connecting line ought to be examined. Where two atolls or reef-encircled islands stand very near each other, the depth between them might be attempted by deep soundings: the bottom has been struck between some of the Maldiva atolls. Generally the form and nature of the reefs encircling islands ought to be compared in every respect with the annular reefs forming atolls.

On the shores of every kind of reef, especially of atolls and of land encircled by barrier reefs, evidence of the slow sinking of the land should be particularly sought for; for instance, by stumps of trees, the foundation-posts of sheds, by wells or graves or other works of art, now standing beneath the level of high-water mark, and which there was good reason to believe must have once stood above its level. The observer must bear in mind that cocoa-nut trees and mangroves will grow in salt-water. If such evidence be found, inquiry

ought to be made whether earthquakes have been felt. On the other hand, all masses of coral standing so much above the level of the sea that they could not have been thrown up by the breakers during gales of wind, at a period when the reef had not grown so far out seaward, should be investigated and their height measured. There is reason to believe that some coralreefs have been thought to have been upraised, owing to the effect of the lateral or horizontal extension of the reefs having been overlooked; for the necessary result of this outward growth is gradually to break the force of the waves, so that the rocks, now further removed from the outer breakers, become worn to a less height than formerly, and the more inland corals not being any longer constantly washed by the surf, cease to live at a level at which they once flourished. It is indispensable that specimens of all upraised corals, and especially of the shells generally associated with them, should be collected; for there can be no doubt that ancient strata containing corals, have in some instances been confounded with recent coral-rock. The importance of ascertaining whether coral-reefs have undergone, or are undergoing, any change of level, depends on the belief that all the characteristic differences between Atolls and Encircling reefs on the one hand, and Fringing reefs on the other, depend on the effect produced on the upwardly-growing corals by the slow sinking or rising of their foundations.

A thick and widely-extended mass of upraised recent coral rock has never yet been accurately examined, and a careful description of such a mass-especially if the area included a central depression, showing that it originally existed as an atoll-is a great desideratum.

Of what nature is the coral-rock; is it regularly stratified or crossed by oblique layers; does it consist of consolidated fine detritus or of coarse fragments, or is it formed of upright corals embedded as they grew? Are many shells or the bones of fish and turtle included in the mass, and are the boring kinds still in their proper positions? The thickness of the entire mass and of the principal strata should be measured, and a large suite of specimens collected.

In conclusion, it may be re-urged that the young geologist must bear in mind, that to collect specimens is the least part of his labour. If he collect fossils, be cannot go wrong; if he be so fortunate as to find the bones of any of the higher animals, he will, in all probability, make an important discovery. Let him, however, remember that he will add greatly to the value of his fossils by labelling every single specimen, by never mingling those from two formations, and by describing the succession of the strata whence they are disinterred. But let his aim be higher: by making sectional diagrams as accurately as possible of every district which he visits (nor let him suppose that accuracy is a quality to be acquired at will), by collecting for his own use, and carefully examining numerous rockspecimens, and by acquiring the habit of patiently seeking the cause of everything which meets his eye, and by comparing it with all that he has himself seen or read of, he will, even if without any previous knowledge, in a short time infallibly become a good geologist, and as certainly will he enjoy the high satisfaction of contributing to the perfection of the history of this wonderful world.

SECTION VII.

ON OBSERVATION OF

EARTHQUAKE PHENOMENA.

BY R. MALLET, A.B., Mem. Ins. C.E., M.R.I.A.

WHENEVER a blow or pressure of any sort is suddenly applied, or the passive force of a previously steady or slowly variable pressure is suddenly either increased or diminished, as these affect material substances, all of which, whether solid, liquid, or gaseous, are more or less elastic, then a pulse or wave of force, originated by such an impulse, is transferred, through the materials acted on, in all directions from the centre of impulse, or in such directions as the limits of the materials permit. The transfer of such an elastic wave is merely the continuous forward movement, of a change in the relative positions, of the integrant molecules or particles, of a determinate volume, affecting in succession the whole mass of material.

Ordinary sounds are waves of this sort in air. The shaking of the ground felt at the passage of a neighbouring railway-train is an instance of such waves in solid ground or rock. A sound heard by a person under water, or the shock felt in a boat lying near a blast exploded under water, are examples of an elastic wave in a liquid.

The velocity with which such a wave traverses, varies in different materials, and depends principally in any given one upon the degree of elasticity, and upon the density. This transit period is constant for the same homogeneous material, and is irrespective of the amount or kind of original impulse: for example, in air its velocity is about 1140-in water about 4700 --and in iron probably about 11,100 feet per second -all in round numbers. In crystallised or pseudocrystalline bodies, such as laminated slate or other rocks, the transit period may vary in three different directions. A very great retardation of this period is produced in solids whose mass is shattered or broken, even when the fissures appear perfectly close.

Thus, if one stand upon a line of railway near the rail, and a heavy blow be delivered at a few hundred feet distant upon the iron rail, he will almost instantly hear the wave through the iron rail-directly after he will feel another wave through the ground on which he stands-and, lastly, he will again hear another wave through the air; and if there were a deep sidedrain to the railway, a person immersed in the water would hear a wave of sound through it, the rate of transit of which would be different from any of the others-all these starting from the same point at the

same moment.

The size of such a wave-that is, the volume of the displaced particles of the material in motion at once, depends upon the elastic limits of the given substance, and upon the amount or power of the original impulse. By the elastic limit in solids is meant the extent to which the particles may be relatively displaced without fracture or other permanent alteration: thus glass,