CHEMICAL AND PHYSICAL CHAIIACTERISTICS OF ROCKS. 417 
most striking of these differences were, of course, noticed even in the 
least civilized times ; and, as observation advanced, and experience 
accumulated, tbo minor and less obvious variations of constitution and 
texture were gradually reduced to order, and classified into the science 
of Mineralogy. 
II. So far our science concerned itself merely with what we should 
now call a correct description of the mineral character of rocks, without 
entering into the question of their genesis or mode of origin. But at 
the dawn, of geology — that is, when mea first conceived that the various 
rocks and strata at the surface of our earth were formed at different, 
and often immeasurably remote periods — then the study of rocks 
entered on a new phase. The age and succession of these formations 
were judged, in the first place, from the order of their superposition ; 
and when it was found, as a general rule, that each formation, or stage 
of this succession, was marked by rocks of a specific mineral character, 
it was hastily concluded that this mineral character was typical of their 
age, and that the latter could be deduced from the former : nor was this 
generalization devoid of broad principles of truth ; the ancient and more 
recent formations are each undoubtedly composed of rocks of very 
different mineral character — the former being made up such of rocks as 
granite and the so-called primary limestones and clay-slates, while the 
latter consists of clays, chalk, or slightly consolidated sandstones 
At any rate this notion served its day, was not without its advantages, 
and was particularly useful in the impetus which it gave to the careful 
study of the mineralogical character of rocks. 
III. "We consequently find in most geological works published in the 
early years of the present century, a full description and classification 
of all the principal rocks, and also a mineralogical description of the rocks 
constituting every formation treated of. But after "William Smith's disco- 
* Recent experience has shown what vei-y erroneous conclusions as to the age of 
rocks we may draw from mineral character alone. " The hillocks of slightly 
coherent mud, marl, and sand, near St. Petersburg, are truly of the same age as 
the slaty mountains of North Wales,'' although they differ little in mineral 
character from the London tertiaries ; while on the flanks of the Rigi, in the Alps, 
at the height of 8,000 or 9,000 feet, "deposits formed at the same time as our 
slightly consolidated London clay have been in many parts converted into schists 
and slates, as crystalline as many of the so-called primary rocks of our islands." — 
"Siluria," pp. 18, 503. Still those arc exceptions ; in nine cases out of ten, the 
crystalline rocks will be found to belong to the old formations, and the slightly 
consolidated to the new. 
