
 ParrIl.§i.] CONSTITUTION OF EARTH'S CRUST. 57 
built up of about sixteen elements, which may be arranged in the two 
following groups, the most abundant bodies being placed first in each 
di . 7 
-_— 

Metalloids. Atonue _ Metals, Misaae 
Weight Weight 
Me SO ee et fone) | Ataminmm . 6 20-30 
peeeeamit st 45) 69488 |, 5-28: 00 @aletum, 5% he, Ae BS SS IGO 
Mearnent pes oe oct ss 5 oe 11°97 Maenesium:..2 2.0 oe $55) 27 ~ 28704 
SOMME c-". ..  BI*98 Potassimm oo a 6s. gs ore ao OF 
Pigerocen =. 5. a 1-00 Sodrams. 36°. c= 4 ee see ee 
Milerites 2. 1). 3) 0 85°87 Tromiraielk. Aire. 2) as ea Soe 
Emosphorus. ....- ..'/- «30:96 Manganese «2... et 84°80 
OE et ae 1. OO LO ATU ee es es teh OOO 
The sixteen elements here mentioned form about ninety-nine parts 
of the earth’s crust; the other elements constitute only about a 
hundreth part, though they include gold, silver, copper, tin, lead, 
and the other useful metals, iron excepted. By far the most 
abundant and important element is Oxygen. It forms about 23 per 
cent. by weight of air, 88°87 per cent. of water, and about a half 
of all the rocks which compose the visible portion or crust of the 
globe. Another metalloid, Silicon, always united with oxygen, ranks 
next in abundance as a constituent of the crust. Of the remaining 
-metalloids, Carbon and Sulphur sometimes occur in the free state, but 
usually in combination with oxygen or some metal. Chlorine (save per- 
haps at volcanic vents) does not occur in a free state, but is abundant 
In combination with the alkalies, especially with sodium. Fluorine 
is always found in combination, and has never yet been isolated by 
artificial chemical processes. It is the only element which has not 
been combined with oxygen. It chiefly occurs in union with Calcium 
as the mineral fluor-spar; but traces of its presence have been 
detected in other minerals, in sea-water, and in the bones, teeth, 
blood and milk of mammalia. Hydrogen occurs chiefly in combin- 
ation with oxygen as the oxide, water, of which it forms 11°13 per 
cent. by weight; also in combination with carbon as the hydro- 
carbons (mineral oils and gases), produced by the slow decomposition 
of organic matter. Phosphorus occurs with oxygen principally in 
calcic phosphate. Of the metals, a few are found in the native state 
(gold, silver, copper, &c.), but those of importance in the framework 
of the earth’s crust have entered into combination with metalloids 
or with each other. 
So far as accessible to observation, the outer portion of our planet 
consists mainly of metalloids. Its metallic constituents have already 
in great part entered into combination with oxygen, so that the 
atmosphere contains the residue of that gas which has not yet united 
itself to terrestrial compounds. In a broad view of the arrangement 
of the chemical elements in the external crust, the suggestive 
speculation of Durocher deserves attention. He regarded all rocks 
as referable to two layers or magmas co-existing in the earth’s crust 
1 Ann, des Mines, 1857, Translated by Haughton, Manual of Geology, 1866, p. 16. 
