418 



GEOLOGY, EXPERIMENTAL. 



verted into hydrated silicates of the type called 

 zeolites, minerals which occur in the cavities 

 of volcanic rocks and sometimes in mineral 

 veins. Another product found in the walls of 

 the glass was the common constituent of rock 

 formations, the anhydrous silicate which is 

 called augite. But the most interesting result 

 was the occasional production of silica in a 

 crystalline form, with all the characteristics 

 of quartz. The quartz usually obtained in the 

 laboratory is a pasty substance which dries up 

 into an amorphous powder. The silica ob- 

 tained by Daubr6e was perfectly crystallized, 

 and corresponded in every respect to quartz 

 crystals, except that, owing to the short dura- 

 tion of the experiment, the crystals were ex- 

 tremely minute in size. 



The pressure and the heat under which many 

 of the geological changes of the earth have 

 occurred can not be repeated artificially in 

 physico-chemical experiments. Another con- 

 dition whicli the experimentalist is still less 

 able to compass is the secular duration of the 

 chemical processes of nature : if all the natu- 

 ral conditions could be artificially produced, 

 the mineral transformations could not be ob- 

 served, as in most instances the chemical action 

 must be prolonged far beyond the life of man. 

 Fortunately, however, this important element 

 has been supplied in certain instances, and we 

 are able to read the results of experiments 

 which, none the less fruitful because acciden- 

 tally begun, have extended over definite periods 

 of many centuries. Most valuable opportuni- 

 ties for watching mineral changes are offered 

 by the thermal springs used by the Romans, in 

 which various mineral substances have been 

 left exposed to the action of the heated water 

 for sixteen or eighteen centuries. The chemi- 

 cal decompositions and combinations which 

 have resulted afford an invaluable experimen- 

 tal illustration and revelation of the process by 

 which mineral lodes or veins are formed. The 

 richest discoveries of this nature were made in 

 the excavations of the hot springs at Bour- 

 bonne-les-Bains, in the department of Haute- 

 Marne in France. In the deposits at the bot- 

 tom of the old Roman baths nearly five thou- 

 sand Roman coins, most of them of bronze, 

 but three or four of gold, and a considerable 

 number of silver, were unearthed, and also 

 pins and rings, statuettes, and other objects in 

 bronze, gold, lead, and iron. Underneath the 

 mud containing these metallic relics was a lay- 

 er of conglomerate formed of sand and frag- 

 ments of sandstone cemented together by min- 

 eral substances which had been produced by 

 the solvent action of the water, heated to a 

 constant temperature of from 58 to 68 C., 

 on the metallic objects during the sixteen cen- 

 turies that they had been exposed in the spring. 

 These products were identical with the min- 

 erals which occur in veins, and in numerous 

 cases they had taken on perfect crystalline 

 forms indistinguishable from those of like sub- 

 stances occurring in natural veins. A great 



variety of these products were detected in the 

 agglutinated mass. One of the most common 

 was cuprite, or red oxide of copper, occurring 

 in octahedral crystals, which was one of the 

 products of the bronze. Chalcosite, or sul- 

 phide of copper, in well-formed crystals, was 

 also found. The yellow copper ore (copper 

 pyrites, the ordinary ore of the copper-mines), 

 the purple copper ore. in which the crystalli- 

 zation is more perfect, and the gray copper 

 ore, a rare mineral which occurs in beautiful 

 tetrahedric crystals, and is thence called tetra- 

 hedrite, were all three present. The leaden 

 pipes of the ancient springs yielded a similar 

 list of products, including the ordinary miner- 

 als contained in lead- veins, and also some rare 

 varieties of minerals, among which may be 

 mentioned phosgenite. From the decomposi- 

 tion of the iron resulted pyrites and other fer- 

 ruginous substances. Even the bricks and con- 

 crete of the masonry were wrought upon by 

 the action of the hot water continued through 

 so long a period, and yielded various silicates 

 which are identical with native minerals. 



An important but thus far unsolvable prob- 

 lem in geology is that of the formation of the 

 vast quantities of limestone rock in which there 

 is no trace of organic structure, but which are 

 made up of fine granular particles of carbonate 

 of lime. They may be formed of the decom- 

 posed substance of shells, but in the absence 

 of structural remains the supposition is equally 

 warrantable that they are merely chemical de- 

 posits of grains of carbonate similar to the 

 concretions formed in hot calcareous springs, 

 which are called in Germany Sprudelstein, 

 and to the species of limestone rock called 

 oolite. If the rock were formed simply by tho 

 precipitation of granular particles of carbonate 

 of lime, the question would remain whether or 

 not the material was derived from the decom- 

 position of older rocks which had been built 

 up of organic remains. The researches of tha 

 English geologist Sorby into the nature of 

 limestone rocks belong to the department of 

 experimental geology. It is possible to distin- 

 guish by their different density and hardness, 

 and by their different optical properties one 

 being optically biaxial, or having two direc- 

 tions in which double refraction does not take 

 place, and the other transmitting the ray un- 

 divided in one direction only between the 

 two forms of the dimorphous crystallization 

 of carbonate of lime, even when the external 

 form is hidden. Sorby shows that it is an 

 important consideration regarding the preser- 

 vation of shells in a fossil form, whether they 

 are composed of carbonate of lime crystallized 

 in the dimorphic form of calcite or in that of 

 aragonite. The latter is an unstable form, 

 whose particles are easily disturbed, and under 

 the action of heat tend to recrystallize in the 

 form of calcite. In a rock composed of shells 

 whose composition was of aragonite the traces 

 of organic forms would entirely disappear. 

 The true corals seem to be principally aragon- 



