XIII.] ORIGIN OF CRYSTALLINE ROCKS. 301 



mechanically formed feldspathic, silicious, and argillaceous 

 sediments just mentioned. 



The direct formation of the crystalline schists from an 

 aqueous magma is a notion which belongs to an early period 

 in geological theory. Delabeche in 1834* conceived that they 

 were thrown down as chemical deposits from the waters of the 

 heated ocean, after its reaction on the crust of the cooling 

 globe, and before the appearance of organic life. This view 

 was revived by Daubree in 1860. Having sought to explain 

 the alteration of palaeozoic strata of mechanical origin by the 

 action of heated waters, he proceeds to discuss the origin of 

 the still more ancient crystalline schists. The first precipitated 

 waters, according to him, acting on the anhydrous silicates of 

 the earth's crust, at a very elevated temperature, and at a great 

 pressure which he estimated at two hundred and fifty atmos- 

 pheres, formed a magma from which, as it cooled, were suc- 

 cessively deposited the various strata of the crystalline schists. t 

 This hypothesis, violating, as it does, all the notions which 

 sound theory teaches with regard to the chemistry of a cooling 

 globe, has, moreover, to encounter grave geognostical difficul- 

 ties. The pre-Cambrian crystalline rocks belong to two or 

 more distinct systems of different ages, succeeding each other 

 in discordant stratification. The whole history of these rocks, 

 moreover, shows that their various alternating strata were de- 

 posited, not as precipitates from a seething solution, but under 

 conditions of sedimentation not unlike those of more recent 

 times. In the oldest known of them, the Laurentian system, 

 great limestone formations are interstratified with gneisses, 

 quartzites, and even with conglomerates. All analogy, more- 

 over, leads us to conclude that, even at this early period, life 

 existed at the surface of the planet. Great accumulations of 

 iron-oxide, beds of metallic sulphides and of graphite, exist in 

 these oldest strata, and we know of no other agency than that 

 of organic matter capable of generating these products. [" The 

 presence of graphite, of native iron, and of sulphurets in most 



* Researches in Theoretical Geology, pp. 297 - 300. 



f Etudes et experiences synthetiques sur le metamorphisme, pp. 119- 121. 



