THE STUDY OF CHEMICAL GEOLOGY. 
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been selected with judgment so as to represent the normal rock 
mass. 
Another example, the reverse of the above, may be given : 
when a diorite (an eruptive rock composed of felspar and horn- 
blende) breaks through quartzose strata, the parts of this rock 
in immediate contact with the strata usually absorb more or 
less quartz particles, and become what the lithologist would call 
a syenite * (rock composed of felspar, hornblende, and quartz), 
and he at once would describe what he would represent as a 
passage of the diorite into syenite. Studying the eruption as a 
whole, the petrologist would have no hesitation in mapping it 
all as diorite, and the microscopic examination of the quartz- 
iferous portion would show him that the quartz had been 
entangled in, but had not crystallised out of, or in other words, 
not been an original constituent of the erupted rock mass. 
Should such a hand specimen, however, be handed over to the 
chemist for analysis, his results, although perfectly exact in 
themselves, would totally misrepresent the composition of the 
rock mass under consideration. 
Such analyses may be of considerable value, when it is known 
that they are intended to represent the changes produced in 
rocks at points of contact with others, or to show the effects of 
weathering, &c., but may greatly mislead when this is not 
expressly pointed out to the geologist, who desires to utilise 
them in his reasonings. 
The backward state of petrological knowledge, especially as 
to the chemical and mineralogical composition of rocks, is in 
great measure due to the method pursued in its study : in 
general the field geologist, quite unacquainted with chemistry, 
and who most probably has never paid attention even to the 
difference between petrology and lithology, on encountering 
a rock in the field knocks off any projecting corner or knob 
which may fall most convenient to his hammer, and sends it to 
the chemist for analysis. How far a geologist not versed in 
chemistry may be subsequently able to appreciate and utilise 
the results of the analysis returned to him by the chemist, is 
open to enquiry ; but it may be safely predicted, that, either 
from proximity to neighbouring rocks of different character, or 
from the decomposition and alteration produced by atmospheric 
* The word syenite,” as used by the English and French geologists, de- 
notes a rock in which quartz is an essential component, along with felspar 
and hornblende. The Germans, however, apply the term syenit ” to what 
the English and French regard as greenstone or diorite, i.e. not containing 
quartz as a normal constituent, but composed of felspar and hornblende 
alone. This must always be remembered when reading German geological 
works, but has unfortunately been in general overlooked by translators. 
