GEOLOGY, EXPERIMENTAL. 



419 





it . The greater number of the shells of gas- 

 tropods are composed of the same ; while the 

 shells of brachiopods are invariably built from 

 oalcite. Some shells show no trace originally 

 of organic structure. Such is the case of the 

 tube of the teredos. In all shells composed of 

 aragonito the organic forms would become ef- 

 faced. The absence of corals or other such 

 shells in limestone formations is therefore no 

 evidence that such organisms did not enter 

 into the original composition of the rock. The 

 preservation of calcite remains, such as the 

 shells of sea-urchins, and the rarity or entire 

 absence of corals and other fossils of aragonite 

 composition, do not prove that the latter did not 

 exist in large numbers, since, their substance 

 being in a state of unstable chemical equilib- 

 rium, the latter would have fewer chances to 

 be preserved. 



The metal lithium was supposed until quite 

 recently to be one of the rarest of all minerals. 

 By the aid of the spectroscope chemists have 

 recently established its presence in a consider- 

 able variety of mineral compounds in feld- 

 spars and micas, in the ashes of tobacco and 

 a number of other plants, and quite abundantly 

 in many mineral springs. Dieulafait, a French 

 geologist, has added to the list of substances 

 unexpectedly containing lithium. He finds it 

 in all the primordial rocks, and also in the 

 waters of the ocean and inland seas. It was 

 before detected in sea-water by Bunsen ; but 

 Dieulafait finds it so diffused as to show its 

 presence in a cubic centimetre of water from 

 the Mediterranean, and in the waters of the 

 Atlantic and the Indian Oceans, the Red Sea, 

 the China Sea, and the Arctic and Antarctic 

 Oceans. In certain mineral springs he detects 

 it in a single drop of the water. He concludes 

 that the salt lithia is no less frequent in min- 

 eral waters, and in the mineral kingdom gen- 

 erally, than potash, soda, and the other kindred 

 alkalies, though present in smaller quantities. 



On the great and mysterious question of the 

 condition of the interior of the earth, the exper- 

 imental method has been appealed to, and the 

 latest theories are chiefly based upon actual ob- 

 servations of the behavior of substances analo- 

 gous to the mineral constituents of the earth 

 during solidification. The theory that the cen- 

 tral nucleus is a solid and not a molten mass is 

 founded on the experiments of Bunsen, Hop- 

 kins, and others, which seemed to prove that 

 silicious substances contracted during solidifica- 

 tion. From this fact the deduction would be 

 evident that pressure would counteract the ef- 

 fect of heat to keep them in a liquid state, that 

 their melting-point would be raised by pressure, 

 and that they would be solid under pressure at 

 temperatures where they would fuse when al- 

 lowed to expand. The question as to the liquid 

 or solid state of the central portions of the globe 

 would then depend on the extent of the con- 

 traction of the materials at solidification and 

 the relative degrees of heat and pressure to 

 which they are subjected. Without such con- 



traction the solidification of the earth would 

 commence at the surface, the masses which 

 first hardened and sank into the liquid maM 

 fusing again as they descended, until the whole 

 became sufficiently viscous to prevent the so- 

 lidified portions from sinking; then a solid 

 crust would form over the whole surface, 

 which would become thicker and thicker as 

 the cooling process went on, until the globe 

 parted with all its internal heat, and became 

 entirely refrigerated. But if the melting-point 

 of the materials were elevated under pressure 

 to such a degree that solid portions sinking 

 into the liquid mass would not fuse, the con- 

 clusron would be that at the center, where the 

 pressure was greatest, the process of solidifica- 

 tion commenced, and extended gradually out- 

 ward, another solidification taking place at the 

 surface at the same time, perhaps. 



The calculations of Sir William Thomson 

 on this subject were based on experiments of 

 Bischoff, which went to show that solid rocks 

 are twenty per cent, denser than the same ma- 

 terials in a molten state. Yet Mallet found 

 that the blast-furnace slags contract only six 

 per cent. Mr. Siemens has recently published 

 a series of experiments made by his brother, 

 which reconcile the discrepancies between 

 these different results, and which, with his ar- 

 guments based on the phenomena of volcanic 

 eruptions, furnish a strong body of proof in 

 favor of the old doctrine that the interior of 

 the globe is filled with a semi-fluid mass of 

 molten rock. Friedrich Siemens found that 

 when glass fused to a thin liquid was allowed 

 to cool, the contraction was marked and rapid 

 until it approached a plastic, viscous consis- 

 tence ; but that after that the contraction was 

 slight in amount, and became less and less ; and 

 that at the point of solidification a slight reSx- 

 pansion seemed to take place. Mr. Siemens in- 

 sists on the impossibility, on Sir William Thom- 

 son's hypothesis of a solid nucleus, of account- 

 ing for the sedimentary strata, many thousand 

 feet in thickness at the earth's surface. The 

 theory of a solid globe fails also to account for 

 the eruptions of volcanoes, since the assump- 

 tion of pockets of liquefied lava existing at dif- 

 ferent depths below the surface does not ex- 

 plain the overflow of lava at the surface. Sie- 

 mens assumes that the hydrous and alkaline 

 lavas which are discharged from volcanoes are 

 less dense than the silicates with which they 

 are associated in the interior of the earth, and 

 than the solid materials of the earth's crust; 

 and that they are forced into the cavities and 

 narrow fissures of the crust by hydrostatic pres- 

 sure, and, when the channel communicates with 

 the surface, are forced out with a pressure 

 which is increased by the inclosed vapor and 

 gases, unless the height of the column is so 

 great that hydrostatic equilibrium is attained 

 before the lava reaches the surface. In har- 

 mony with many other recent thinkers, Sie- 

 mens supposes the ocean-beds and lower por- 

 tions of the earth's surface to be composed of 



