iF 7 x <-> ee 4a Ee nets ip Kae Pe ae ee fe 
‘\ 
56 ct OG MOS. os | [Book IL. 
Wot now bein all respects like the earth, so far as we know it at 
present.” * : | 
(3.) The third kind of evidence leads to results confessedly less 
emphatic than those from the two previous lines of reasoning. It 
is based upon calculations as to the amount of heat that would be 
available by the falling together of the masses from space, which 
gave rise by their impact to our sun, and the rate at which this heat 
has been radiated. Assuming that the sun has been cooling even 
at a uniform rate, Professor ‘I'ait comes to the conclusion that it 

cannot have supplied the earth, even at the present rate, for more 
than about 15 or 20 million years.” 
= 
Part I1.—An Account oF THE COMPOSITION OF THE FEARTH’S 
CRUST—MINERALS AND Rooks. . 
The earth’s crust is composed of mineral matter in various aggre- 
gates included under the general term Rock. A rock may be 
defined as a mass of matter composed of one or more simple minerals, 
haying usually a variable chemical composition with no necessarily 
symmetrical external form, and ranging in cohesion from mere loose 
débris up to the most compact stone. Granite, lava, sandstone, lime- 
stone, gravel, sand, mud, soil, marl and peat, are all recognized in a 
geological sense as rocks. 
It will be most convenient to treat—Ist, of the general chemical 
constitution of the crust; 2nd, of the minerals of which rocks mainly 
consist ; 3rd, of the external characters, and, 4th, of the internal tex- 
ture and structure, of rocks; 5th, of the classification of rocks; 6th, 
of the more important rocks occurring as constituents of the earth’s 
crust ; and 7th, of the methods employed for their-determination, 
§ i. General Chemical Constitution of the Crust. 
Direct acquaintance with the chemical constitution of the globe 
must obviously be limited to that of the crust, though by inference 
we may eventually reach highly probable conclusions regarding the 
constitution of the interior. Chemical research has discovered that 
sixty-four® simple oras yet undecomposable bodies, called elements, in 
various proportions and compounds, constitute the accessible part of 
the crust. Of these, however, the great majority are comparatively 
of rare occurrence. The crust, so far as we can examine it, is mainly 
' Trans. Geol. Soc. Glasgow, iii. p. 16. Professor Tait, in repeating this argument 
concludes that, taken in connection with the previous one, “it probably reduces the 
possible period which can be allowed to geologists to something less than 10 millions of 
years,” Op. cit, p. 174. 
: Op. cil, p. 174. 
* This number has within the last two years been increased by the alleged discovery 
of no fewer than fourteen new metals. Some of these bodies however, have not yet 
been satisfactorily proved to be new. T. 8. Humpidge, Nature xxii. p. 232. 
