370 D. Forbes — On the Study of Chemical Geology. 



The backward state of petrological knowledge, especially as to the 

 chemical and mineralogicaLcomposition of rocks, is in great measure 

 due to the method pursued in its study : in general the field geolo- 

 gist, quite vmacquainted with chemistry, and who most probably has 

 never paid any 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 geo- 

 logist not versed in chemistry may be subsequently able to appre- 

 ciate 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 

 influences or weathering on all surface-rocks, that hand-specimens 

 so collected are not likely to turn out correct representatives of the 

 rock-mass as a whole. 



When it is proposed to make a chemical examination of any parti- 

 cular rock, it should first of all be carefully studied in the field, in 

 order that a correct opinion may be formed as to the true nature of 

 the rock-substance itself, when uncontaminated or unchanged by 

 external influences ; a specimen may then be taken which, in some 

 measure, will represent the actual rock-mass on the large scale, al- 

 though this is attended with considerable difficulty and trouble, un- 

 less (as fortunately in England is generally the case) excavations or 

 quarries have laid bare a face of rock, and so afforded facilities for 

 obtaining the unaltered rock itself. 



The quantity required to be taken and pulverised, in order to 

 obtain an average for analysis, must entirely depend ujDon whether 

 the rock is of a fine or coarse grained texture. In the latter case, a 

 much larger quantity must naturally be employed. 



All rock examinations should include a careful description of the 

 mineral constituents of the rock itself, which, if fine-grained or com- 

 pact, can only be effected by making a section and submitting it to 

 the microscope. The physical properties, as specific gravity, etc., 

 should also be noted, as well as, of course, the relations of the rock 

 to the general geology of its district, and the occurrence of any 

 accessory minerals disseminated in it. 



It has been considered necessary to lay great stress upon all points 

 connected with the selection and analysis of rocks, for, if these be 

 not attended to, the labour bestowed upon them may be regarded as 

 entirely thrown away. It cannot but be admitted that the possession 

 of a series of accurate and trustworthy analyses of rocks is of almost 

 vital importance to the advancement of Chemical Geology ; and it is 

 sincerely to be hoped that, considering the backward state of our 

 knowledge of this subject in England, some efforts will be made to 

 remedy tliis defect, and so provide correct data for advancing further 

 research into this promising department of geology. 



