52 



NATURE 



{_Nov. 1 6, 1871 



somewhat elevated temperature under pressure. I showed that 

 what he regarded as dolomite was not such, but an admixture of 

 carbonate of lime with anhydrous and sparingly soluble carbonate 

 of magnesia ; the conditions in which the carbonate of magnesia 

 is llber.ited in this reaction not being favourable to its union with 

 the carbonate of lime to form the double salt which constitutes 

 dolomite. The experiment of Marignac, who thought to form 

 dolomite by substituting a solution of chlorid of magnesium for 

 the sulphate, I found to yield similar results, the greater part of 

 the magnesian carbonate produced passing at once into the in- 

 soluble condition, without combining with the excess of carbo- 

 nate of lime present. The process for the production of the double 

 carbonate described by Ch. Deville, namely, the action of vapours 

 of anhydrous magnesian chlorid on heated carbonate of lime, in 

 accordance with Von Buch's strange theory of dolomitisation, I 

 have not thought necessary to submit to the test of experiment, 

 since the conditions required are scarcely conceivable in nature. 

 Multiplied geognostical observations show that the notion of the 

 epigenic production of dolomite from limestone is untenable, al- 

 though its resolution and deposition in vein.s, cavities, or pores in 

 other rocks is a phenomenon of frequent occurrence. 



The dolomites or magnesian limestones may be conveniently 

 considered in tw o classes ; first, those which are found with 

 gypsums at various geological horizons ; and secondly, the more 

 abundant and widely distributed rocks of the same kind, which 

 are not associated with deposits of gypsum. The production of 

 the first class is dependent upon the decomposition of sulphate of 

 magnesia by solutions of bicarbonate of lime, while thoie of the 

 second class owe their origin to the decomposition of magnesian 

 chlorid or sulphate by solutions of alkaline bicarbonates. In both 

 cases, however, the bicarbonate cf magnesia, which the carbonated 

 waters generally coiilain, contributes a more or less important 

 ])art to the generation of the magnesian sediments. The car- 

 bonated alkaline waters of deep-seated springs often contain, as is 

 well known, besides the bicarbonates of soda, lime and magnesia, 

 compounds of iron, manganese, and many of the rarer metals in 

 solution, and thus the metalliferous character of many of the 

 dolomites of the second class is explained. The simultaneous 

 occurrence of alkaline silicates in such mineral waters, would 

 give rise, as already pointed out, to the production of insoluble 

 silicates of magnesia, and thus the frequent association of such 

 silicates with dolomites and magnesian carbonates in the crystalline 

 schists is explained, as marking poitions of one continuous pro- 

 cess. The formation of these mineral waters depends upon the 

 decomposition of feldspathic rocks by subterranean or sub aerial 

 processes, which were doubtless more active in former ages than 

 in our own. The subsequent action upon magnesian waters of 

 these bicarbonated solutions, whether alkaline or not, is depen- 

 dent upon climatic conditions, since, in a region where the rain- 

 fall is abundant, such waters would find their way down the 

 river-courses to the open sea, where the excess of dissolved sul- 

 phate of lime would prevent the deposition of magnesian car- 

 bonate. It is in dry and desert regions, with limited lake-basins, 

 that we must seek for the production of magnesian carbonates, 

 and I have argued from these considerations that much of north- 

 eastern America, including the present basins of the Upper 

 Mississippi and .St. Lawrence, must, during long intervals, in the 

 palreozoic period, have had a climate of excessive dryness, and a 

 surface marked by shallow enclosed basins, as is shown by the 

 widely-spread magnesian limestones, and the existence of gypsum 

 and rock-salt at more than one geological horizon within that 

 area. * The occurrence of serpentine and diallage at Syracuse, 

 New York, offers a curious example of the local development of 

 crystalline magnesian silicates in Upper Silurian dolomitic strata 

 under conditions which are imperfectly known, and which, in the 

 present state of the locality, cannot be studied ,+ 



Since the uncombinedand hydrated magnesia mono-carbonate 

 is at once decomposed by sulphate or chlorid of calcium, it fol- 

 lows that the whole of these lime-salts in a sea-basin must be 

 converted into carbonates before the production of carbonated 

 magnesian sediments can begin. The carbonate of lime formed 

 by ihe action of carbonates of magnesia and soda, remains at 

 first dissolved as bicarbonate, and is only separated in a solid 

 form, when, in excess, or when required for the needs of living 

 plants or animals, which are dependent for their supply of 

 calcareous matter, on the bicarbonate of lime produced, in part 

 by the process just described, aird in part by the action of car- 



* Geology of South-western Ontario, Amer. Jour.Sci. 11. xlvi. 355. 

 t Geology'of the 3rd district of New York, loSllo, and Hunt onOphiolites, 

 Amer, Jour. Sci. 11. xxvi. 236. 



bonic acid on insoluble lime-compounds of the earth's solid crust 

 So many limestones are made up of calcareous organic remains, 

 that a notion exists among many writers on geology that all lime- 

 stones are, in some way, of organic origin. At the bottom of 

 this lies the idea of an analogy between the chemical relations of 

 vegetable and animal life. As plants give rise to beds of coal, 

 so animals are supposed to produce limestones. In fact, how- 

 ever, the synthetic process by which the growing plant, from 

 the elements of water, carbonic acid and ammonia, generates 

 hydrocarbonaceous and azotised matters, has no analogy with the 

 assimilative process by which the growing animal appropriates 

 alike these organic matters and the carbonate and phosphate of 

 lime. Without the plant, the synthesis of the hydrocarbons 

 would not take place, while independently of the existence of 

 coral or moUusk, the carbonate of lime would still be generated 

 by chemical reactions, and would accumulate in the waters until, 

 these being saturated, its excess would be deposited as gypsum 

 or rock-salt are deposited. Hence, in such waters, where, from 

 any causes, life is excluded, accumulations of pure carbonate of 

 lime may be formed. In 1S61 I called attention to the white 

 marbles of Vermont, which occur intercalated among impure 

 and fossiliferous beds, as apparently examples of such a process.* 



It is by a fallacy similar to that which prevails as to the 

 organic origin of limestones, that Daubeny and Murchison were 

 led to appeal to the absence of phosphates from certain old 

 strata as evidence of the absence of organic life at the time of 

 their accumulation, t Phosphates, like silica and iron-oxyd, were 

 doubtless constituents of the primitive earth's crust, and the pro- 

 duction of apatite crystals in granitic veins, or in crystalline 

 schists, is a process as independent of life as the formation of 

 crystals of quartz or of hematite. Growing plants, it is true, 

 take up from the soil or the waters dissolved phosphates, which 

 passed into the skeletons of animals, a process which has been 

 active from very remote periods. I showed in 1S54 that the 

 shells of Lingula and Orbicula, both those from the base of the 

 paljeozoic rocks and those of the present time have (like Conu- 

 laria and Serpulites) a chemical composition similar to the skele- 

 tons of vertebrate animals, t The relations of both carbonate 

 and phosphate of lime to organised beings are similar to those of 

 silica, which, like them, is held in watery solution, and by pro- 

 cesses independent of life is deposited both in amorphous and 

 crystalline forms, but in certain cases is appropriated by diatoms 

 and sponges, and made to assume organised shapes. In a word, 

 the assimilation of silica, like that of phosphate and carbonate 

 of lime, is a purely secondary and accidental process, and where 

 life is absent, all of these substances are deposited in mineral and 

 inorganic forms. 



I have thus endeavoured to sketch, in a concise and rapid 

 manner, the history of the earlier rock-formations of eastern 

 North America, and of our progress in the knowledge of them ; 

 while I have, at the same time, dwelt upon some of the geognos- 

 tical and chemical questions which their study suggests. With 

 the record of the last thirty years before them, American geolo- 

 gists have cause for congratulation that their investigations have 

 been so fruitful in great results. They see, however, at the same 

 time, how much yet remains to be done in the study of the Appa- 

 lachians and of our north eastern coast, before the history of these 

 ancient rock-formations can be satisfactorily written. Meanwhile 

 our adventurous students are directing their labours to the vast 

 regions of western America, where the results which have already 

 been obtained are of profound interest. The progress of these 

 investigations will doubtless lead us to modify many of the views 

 now accepted in science, and cannot fail greatly to enlarge the 

 bound of geological knowledge. 



THE SCOTTISH SCHOOL OF GEOLOGY \ 

 II. 



WHILE Hutton fortified his convictions by constant appeals 

 to the rocks themselves, his disciple Hall tested their truth 

 in the laboratory. It is the boast of Scotland to have led the 

 way in the application of chemical and physical experiment to 

 the elucidation of geological history. It was objected to Hut- 

 ton's theory, that if basalt and similar rocks had ever been in a 



• Amer. Jour. Sci. II. xxxi. 402. 



t Siluria, 4th ed. pp. 28 and 517. 



} Amer. Jour. Sci. II. xvii. 236. 



§ A Lecture delivered .it the opening of the class of Geology and Mineralogy 

 in the University of Edinburgh, by Archibald Geikie, F.R.S., Nov. 6, 1871, 

 concluded from p. 39. 



