D. Forbes, — On the Study of Chemical Geology. 369 



original crust of the globe must be regarded as representing what 

 may be termed the first eruptive or massive silicated rocks. 



The study of the chemistry of the eruptive rocks, becomes, therefore, 

 a subject of special interest and importance, not only as tending to 

 elucidate their origin and formation, but also as bearing on the nature 

 of the sedimentary strata which, as before mentioned, have, directly 

 or indirectly, been formed from their ruins. 



In Great Britain, it must be acknowledged, there is at this present 

 time, little or no information on this subject in print ; and of the few 

 chemical analyses of rocks which have been published, it is to be 

 feared that many of them have been made on specimens which have 

 not been selected with care, so as to represent the actual rock-mass 

 in question ; and therefore such analyses, however accurately done, 

 quite misrepresent the composition of the rock-mass as a whole ; in 

 fact, a litJiological analysis is returned where a petrological analysis is 

 required. 



The terms lithology and petrology are continually misapplied and 

 used for one another, notwithstanding that the difference between 

 them is clearly indicated by their derivations ; petrology from the 

 Greek "Trerpo^, a rock,"^ being the study of rock masses in situ, 

 their relations, occurrence, origin, mineral character, physical struc- 

 ture, chemical composition, etc. ; whilst, on the other hand, lithology 

 from "\l6o<;, a stone," is more properly applied to the consideration 

 of stones or detached mineral masses not in situ, blocks, boulders, 

 pebbles, etc., such as are found in drift-gravel, alluvial formations, 

 conglomerates, etc. A knowledge of lithology may be acquired in 

 the cabinet, but petrology must of necessity be studied in the field. 



The petrologist, by studying rock-masses on a large scale, discovers 

 simplicity in cases where the lithologist would but eliminate con- 

 fusion. By a careful examination of the rock in situ, assisted by the 

 use of his microscope and laboratory, he comes to the conclusion that 

 all the innumerable rock-species of the lithologist do not exist in 

 nature as rocks, but are mere subordinate portions, altered in appear- 

 ance or composition by subsequent influences. 



Such alterations, or transitions as they are often called, are ex- 

 tremely common at the points of contact of sedimentary with 

 eruptive rocks ; thus, for example, a Millstone grit or Carboniferous 

 sandstone may, near the point of contact with an eruptive rock, be 

 found to be lithologically quartzite, similar in appearance to some of 

 even the most ancient quartzites, whilst petrol ogically considered, it 

 is but sandstone. Again, a micaceous sandstone or a mica-schist 

 bed may, at the point of contact with a felspathic eruptive rock, 

 become in mineral composition a gneiss from the absorption of fel- 

 spar, yet it is not so petrologically ; the petrologist does not base 

 his opinion upon mere hand-specimens, unless in the rare cases 

 where they have been selected with judgment so as to represent the 

 normal rock mass. 



^ Also irerpa, whence the synonym Petralogy. In the, for its time, very excellent 

 treatise on rocks — Pinkerton's "Petralogy," 2 vols. London, 1811 — the distinction 

 between Lithology and Petralogy is fully explained. 



