254 
POPULAR SCIENCE EEYIEW. 
wliich, however accurately they may show the chemical compo- 
sition of the stone actually submitted to analysis, may at the 
same time quite misrepresent the composition of the rock mass 
as a whole ; in fact, a lithological anal}^sis 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 ; petro- 
logy from the Greek “ irsrpns^ a rock,” * being the study of 
rock masses in situ, their relations, occurrence, origin, mineral 
character, physical structure, chemical composition, &c., whilst, 
on the other hand, lithology from “ \i6os, a stone,” is more 
properly applied to the consideration of stones or detached 
mineral masses nut in situ, blocks, boulders, pebbles, &c., such 
as are found in drift gravel, alluvial formations, conglome- 
rates, &c. 
A knowledge of lithology may be acquired in the cabinet, 
but petrology must of necessity be studied in the field. From 
the examination of hand specimens, the lithologist coins innu- 
merable names to indicate mere varieties of rocks, or rather 
stones, the very multitude of which inclines him at last to 
believe in all manner of transitions and transmutations of one 
rock into another. 
The petrologist on the contrary however, by studying rock 
masses on the large scale, discovers simplicity in cases where 
the lithologist would but elimiuate confusion. 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 
these innumerable rock species do not exist in nature as rocks, 
but are mere subordinate portions, altered in appearance or 
composition by subsequent influences. 
Such alterations, or transitions as they are often called, are 
extremely common at the points of contact of sedimentary with 
eruptive rocks ; thus, for example, a milstone grit or carboni- 
ferous sandstone may, near the point of contact with an eruptive 
rock, be found to be lithologically quartzite, similar in ap- 
pearance to some of even the most ancient quartzites, whilst, 
petrologically considered, it is but sandstone. Again, a mica- 
ceous sandstone or a mica schist bed may, at the point of 
contact with a felspathic eruptive rock, become in mineral com- 
position a gneiss from the absorption of felspar, 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 
* Also TTF-rpa, whence the synonym Tetralogy. In the, for its time, very 
excellent treatise on rocks — Pinkerton’s Tetralogy,” 2 vols. London : 1811 
— the distinction between Lithology and Tetralogy is fully explained. 
