34 Joty—An Estimate of the Geological Age of the Earth. 
Mr. F. W. Clarke has estimated the percentage amounts of the elements 
contained in the EHarth’s surface crust. In Mr. Clarke’s first report,* the mean of 
880 selected analyses of American and European igneous, volcanic, and crystalline 
rocks is tabulated along with the means of the component analyses divided into 
local groups, as the rocks of the Western States, of Northern California, of 
European volcanic and crystalline rock, &¢.; and it is remarked as the result of 
comparing these groups that “the thesis that the crust of the Earth is fairly 
homogeneous in composition is thus sustained by positive evidence.” In a later 
publication, f 960 analyses are consulted, and these of a still more carefully selected 
and reliable character, giving an average ‘‘ which may fairly represent the compo- 
sition of the older crust of the Earth.” The result, which closely agrees with the 
earlier estimate, is contained in the column below. 
Si0,, ; : : 5 HPT 
IO, ¢ 96 o 0 158 
Fe,0;, ; : Oo 
FeO, : : : . 8:44 
CaO, ; : 5 >  —- BIL 
MgO, : é . 4:40 
K,0, 0 . : 5 OFS 
Na,0, : : : 5 BGil 
H,0, ; 5 , 5 Lil 
TiO., é ; F OLOS 
P50; : ‘ 3 Oil 
99°14 
This approximates to a Diorite, and would fall among Rosenbusch’s series of 
‘“¢ Granito-dioritischen” and ‘‘ Gabbro-peridotitischen ” magmas. 
Such a rock or lava attacked by a heated solution of hydrochloric acid must 
ultimately yield its iron, calcium, magnesium, potash, and soda as chlorides. 
The atomic percentages of Clarke’s average are given by him as follows :— 
Iron, : A ; ee eri 
Calcium, . ‘ ; 5 OS} 
Magnesium, . : - 2°64 
Potassium, ‘ : 5 PSS 
Sodium, . : ; LOS 
The chlorme taken up may be assumed to be distributed as follows: in the 
first instance—Fe,Cl,, CaCl, MeCl,, KCl and NaCl. 
* Bulletin of the United States Geological Survey, No. 78, 1891, p. 34. 
+ Bulletin of the U.S. Geological Survey, No. 148, 1897, p. 12. 
+ Elemente der Gesteinslehre. Stuttgart, 1898, p. 187. See No. 15 of this group for a rough 
approximation to Clarke’s average. 
