Their theory supposes that there are large hollow spaces in the crust of the earth, into which immense falls of material take place, and that these are the cause of a part of the earthquakes; that the crust of the earth is often and variously disturbed in consequence of the constant contraction dependent upon the cooling of the globe. It is broken up into separate masses which in their turn are dislocated horizontally or vertically; is lifted up and folded into immense mountain ranges, the arches of which, breaking, may again suffer dislocation. Thus continuous action in movement of masses and foldings is constantly going on in the earth. Edouard Suess, the distinguished Austrian geologist, has indeed constituted a special earthquake type to correspond with this type of mountain formation. Since, in consequence of this condition, tension is present everywhere in the crust of the earth, it may come to pass that it shall be relieved by a distant earthquake, and another earthquake, which may be called a relay or transmission earthquake, be produced thereby. Hence we have, besides the volcanic, the landfall, the tectonic (in the strict sense), and the transmission earthquakes. The sources of earthquake force lie, then, according to this theory, in the incompleteness of the earth's crust, the effects of gravity, and the earth's loss of heat. And is the supposition not very probable? Do we not see similar processes going on over the whole earth, in the shape of earthquakes, landslides, fissures, subsidences of land, and the like? And as the Alps were lifted up, and the plain of the Rhine was depressed between the Vosges and the Black Forest, may not mightier dislocations, breaches, and destruction occur? Why may not the processes which took place in the earlier epochs of the earth's history and were so powerful in the more recent Tertiary be still going on? All this seems so plausible that, with a few exceptions, the theory has been almost universally agreed in. I briefly mention here Falb's theory, which, accepting the earlier views, ascribes earthquakes to periodical swellings of the fiery fluid interior of the earth, only because of the effect it has had on the public in connection with some wholly unscientific predictions. More worthy of consideration is the theory of Daubrée, the late distinguished master of French and especially Alsatian geology, who did not attribute the similar phenomena of volcanic and nonvolcanic earthquakes to different causes, but maintained that all earthquakes were produced by superheated steam issuing from surface waters. But this theory needs no refutation. There are, however, some serious objections to the tectonic theory of earthquakes, plausible as it may seem. In order to weigh them as we ought, we must as briefly as possible construct a picture of the constitution of the earth's interior.