THE PAST AND FUTURE OF GEOLOGY. 179 



also explains why uuder these conditions their record among the Fraun- 

 hofer lines should be a feeble one. Hence he considers that we have no 

 argument against the presence of some quantity of the metalloids in 

 the sun taken as a whole, although that quantity may be small. 



Mr. Lockyer then takes the observed facts together with the hypoth- 

 esis of the external position of the metalloids, and is considering these 

 two questions : 



1. Assuming the earth to have once been in the same condition as the 

 sun now is, what would be the chemical constitution of its crust? 



2. Assuming the solar nebula to have once existed as a nebulous star 

 at a temperature of complete dissociation, whac would be the chemical 

 constitution of the planets thrown oft' as the nebulosity contracted! 



It will be seen that there is a most intimate connection between these 

 ,two inquiries, the localization of the various elements and the reduction 

 of temperature acting in the same way in both cases. 



Thus, to deal with the first question : as the external gaseous vapors 

 (those of the metalloids) cooled, they condensed and fell on the under- 

 lying layer, where they entered into combination, forming one set of 

 binary compounds and then others as the temperature Avas reduced, 

 until finally all the metals and earths were precipitated.* 



If, now, we turn to the earth's crust, we find it very generally as- 

 sumed that the fundamental igneous rocks which underlie the sedi- 

 mentary strata, and which formed originally the outer layers, may be 

 divided into two great masses, holding a general relation one to the 

 other, an upper one consisting of granite and other plutonic rocks, rich 

 in silica, moderate in alumina, and poor in lime, iron, and magnesia; 

 and of a lower mass of basaltic and volcanic rocks of greater specific 

 gravity, with silica in smaller proportions, alumina in equal, and iron, 

 lime, and magnesia in much larger i)roportions, with also a great variety 

 of other elements as occasional constituents; while the denser metals 

 are in larger proportion in the more central portion of the nucleus. The 

 suggestion of Mr. Lockyer is that this order follows necessarily from 

 the original localization of the earths and metals before referred to, by 

 which the oxygen, silicon, and other metalloids formed, as they now do 

 in the sun, an outer atmosphere, succeeded by an inner one consisting 

 in greater part of the alkaline earths and alkalies, then by a lower one 

 of iron and its associated group of metals, and finally by an inner nu- 

 cleus containing the other and denser metals. 



As we have before observed, above nine-tenths of the earth's crust 

 consists of those elements which, on the assumption of the external posi- 

 tion of the metalloids, would constitute the outer layers of the nebular 

 mass. Thus, oxygen and silicon alone constitute, on the average, y^ 



* FirBtly, those binary componiids capable of existiiip; at a high temperaturt', such 

 as the vapor of water, of hydrochloric acid, of silica, carbonic acid, and others, would 

 be formed; Kticondly, the jjrecipitation of these would jiivcj rise to numerous react ious, 

 formiiijr a variety of silicates, chlorid<!S, sulphates, &c. ; thirdly, with the condensation 

 of water, the constitution of minerals would be t>tVecte<l, <loubl(! decompositions would 

 ensue, and the consolidutiou of the out«r shell commence. 



