A HISTORY OF SCIENCE 



grains and the larger and coarser plates of mica sub- 

 siding first and the finest last. 



" But the fragments of quartz and mica were not 

 deposited alone ; a great proportion of the quartz held 

 in solution must have been precipitated at the same 

 time as the water cooled, and therefore by degrees lost 

 its faculty of so much in solution. Thus was gradually 

 produced the formation of mica-schist, the mica im- 

 perfectly recrystallizing or being merely aggregated 

 together in horizontal plates, between which the quartz 

 either spread itself generally in minute grains or uni- 

 fied into crystalline nuclei. On other spots, instead 

 of silex, carbonate of lime was precipitated, together 

 with more or less of the nucaceous sediment, and gave 

 rise to saccharoidal limestones. At a later period, 

 when the ocean was yet further cooled down, rock-salt 

 and sulphate of lime were locally precipitated in a sim- 

 ilar mode. 



" The fifth stratum was aeriform, and consisted in 

 great part of aqueous vapors; the remainder being a 

 compound of other elastic fluids (permanent gases) 

 which had been formed probably from the volatiliza- 

 tion of some of the substances contained in the primi- 

 tive granite and carried upward with the aqueous 

 vapor from below These gases will have been either 

 mixed .together or otherwise disposed, according to 

 their different specific gravities or chemical affinities, 

 and this stratum constituted the atmosphere or aerial 

 envelope of the globe. 



" When, in this manner, the general and positive ex- 

 pansion of the globe, occasioned by the sudden reduc- 

 tion of outward pressure, had ceased (in consequence 



136 



