106 GEOLOGY. 
The cause of it is, that such parts as become more easily solid, separate first without the force 
of crystallization being active. For this reason there are sometimes considerable differences in 
‘a rock in short distances. The amount of the variable part changes, however, only slightly. 
Taking the mean of a great number of analyses, Bunsen gets the composition of the normal 
trachytic and the normal pyroxenie rocks of Iceland. In the rest, being mixtures of these two, 
the ratio of the oxygen of the acid to that of the bases varies between the given limits. Their 
character depends upon these proportions and the physical conditions that ruled their forma- 
tion. Be 8 the per cent. of silica in such a mixed rock, be s the per cent. of silica in the normal 
trachytic, and ¢ in the normal pyroxenic rock—then Soe uj where a represents the quantity 
of normal pyroxenie rock to be mixed with one part of trachytic rock, in order to give the 
composition of the mixed rock. For the computation of a, B chose the silica, which is easily 
determined, and by the quantity of silica the quantities of ihe other parts are also given. Be 
Por Po £p the quantities of the single component parts in one part of normal pyroxenic, 
and £,, fs t,....¢,, the same in one part of normal trachytic rock, we may then calculate the 
quantities of the component parts in a mixed rock by the equation— 
J D 4: crt) +e 
With the help of this formula si the si sans Fx primitive rocks of Tad have 
been calculated and put in tables, and with these theoretical numbers all the analyses of Ice- 
landic rocks agree perfectly; the law holds equally good for the volcanic rocks of Armenia and 
volcanic rocks generally, and its applicability to the older or plutonic rocks has been put beyond 
all doubt by the analysis of a great number of granites, sienites, diorites, porphyritic and 
hornblende rocks, from different countries. 
The compositions of the normal trachytic and normal pyroxenic rocks are: 
t ; 
Norm. trach. | Norm. pyr. 
raid niin 76. 67 48, 47 
XYd of aluminium 
Prothxyd et in d 14.93 . 80. 16 
d of calcium S 1.44 11. 87 
Oxyd of agnesium 0.28 6. 89 
Oxyd of potassium . 90 0. 65 
Oxyd of sodium 4.18 1, 96 
100. 00 100. 00 
The analysis of a rock from the Sierra Nevada, about 40 miles south of Madelin Pass, gave 
50.93 per cent. of silica. According to the first of the above formulas, this rock is a mixture 
of 1 part of normal trachytic and 10.46 parts of normal pyroxenic rock. The second formula 
gives its composition as— 
NOM cee ws 50.93 
Protoxyd of iron and alumina 98.11 
Oxyd of calcium 10.96 
Magnesia .. 6.31 
Oxyd of sodium and potassium 3.03 
100.00 
mA 
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