CHEMICAL AND PHYSICAL CHARACTERISTICS OF ROCKS. 417 



most striking of these differences were, of course, noticed eren in tlie 

 least civilized times ; and, as obseryation advanced, and experience 

 accumulated, the 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 men 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 Y/iiliam Smith's disco- 



Recent experience has sliOT^n what very erroneous conchisions 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 these are exceptions ; in nine cases out of ten, the 

 crystalline rocks will he found to belong to the old formations, and the slightly 

 consolidated to the new. 



