180 THE PAST AND FUTURE OF GEOLOGY. 



of the mass of acid plutouic rocks of which the upper part of the first 

 assumed shell of the earth consists. Beueath it, as a whole, are the 

 basic rocks, into the composition of which calcium, magnesium, and iron, 

 combined with oxygen, enter in the ratio of, say, -^^\, while the silicon 

 diminishes in proportion. Still dee{)er lie the denser and harder metals, 

 which reach the surface only through veiivs transversing the outer layers. 



We next come to the second question, dealing with the chemical con- 

 stitution of the planets. It is imagined that the same consideration 

 would hold good, and that the exterior planets may approach in their 

 constitution that of the sun's outer atmosphere, and that the planets 

 may become more metaffic as their orbits lie nearer the central portion 

 of the nebula. Mr. Lockyer considers that the low density and gigantic 

 and highly-absorbing atmospheres of the outer planets accords with 

 their being more metalloidal ; and that, on the other hand, the high 

 density and comparatively small and feebly-absorbing atmospheres of 

 the inner planets points to a more intimate relation with the inner layers 

 of tbe original nebulous mass. For the same reason, we should expect 

 to find the metalloids scarcer in the sun than in the earth. 



In the Jovian system, and in our own moon, we have a still further 

 support of the hypothesis in the fact that the density of the satellites 

 is less than that of their primary. 



I had hoped to have brought before you some of the results of the 

 examination of the spectra of portions of the outer igneous-rock crust of 

 the earth, which Mr. Lockyer kindly undertook to compare with the 

 solar spectrum; but, owing to the state of the weather, the investigation 

 is not yet complete. It may be stated, however, that as in the spectrum 

 of the sun so in the spectra of the lava, greenstone, and granite already 

 tested, no trace of the metalloids is present, although oxygen and silicon 

 enter so largely' into the composition of these rocks. 



We can, however, still only look on these views as hypothetical, but 

 they commend themselves to us by their simplicity and grandeur, and 

 their high suggestiveness for future inquiry and research. They show 

 us also how the spectroscope may, as the microscope has done already, 

 aid the investigations of the geologists — the one by endowing the eye 

 ■with new powers of sight with respect to the infinitely minute, and the 

 other with new powders of tangible analysis with respect to the infinitely 

 distant in time and in space. 



Quitting the early history of our globe, we leave the domain of the 

 astronomer and enter upon one shared by the geologist, the mineralo- 

 gist, the chemist, and the mathematician. The elements which we first 

 dealt with in their gaseous and dissociated state have now entered into 

 a multiplicity of combinations giving rise to a vast variety of compound 

 bodies. Instead of t4ie sixty-four simple elements, their mutual reac- 

 tions have resulted in the formation of somewhere about one thousind 

 varieties of rocks and minerals alone, with which the geologist has in 

 futuire to deal. He also has to deal with all the i)hysical problems 

 arising from the consolidation of the crust of the earth, from pressure 



