90 CLASSIFICATION OF ROCKS. [Ch. VIII. 



In the first chapter it was stated that the four great classes of rocks, tha 

 aqueous, the volcauic, the plutouic, and the metamorphic, would each be 

 considered not only in reference to their mineral characters, and mode of ori- 

 gin, but also to their relative age. In regard to the aqueous rocks, we have 

 already seen that they are stratified, that some are calcareous, others argil- 

 laceous or siliceous, some made up of sand, others of pebbles ; that some 

 contain freshwater, others marine fossils, and so forth ; but the student has 

 still to learn which rocks, exhibiting some or all of these characters, have 

 originated at one period of the earth's history, and which at another. 



To determine this point in reference to the fossiliferous formations is 

 more easy than in any other class, and it is therefore the most convenient 

 and natural method to begin by establishing a chronology for these strata, 

 and then to refer as far as possible to the same divisions the several groups 

 of plutonic, volcanic, and metamorphic rocks. Such a system of classifica- 

 tion is not only recommended by its greater clearness and facility of ap- 

 plication, but is also best fitted to strike the imagination by bringing into 

 one view the contemporaneous revolutions of the inorganic and organic 

 creations of former times. For the sedimentary formations are most readily 

 distinguished by the different species of fossil animals and plants which 

 they inclose, and of which one assemblage after another has flourished and 

 then disappeared from the earth in succession. 



But before entering specially on the subdivisions of the aqueous rocks 

 arranged according to the order of time, it will be desirable to say a few 

 words on the chronology of rocks in general, although in doing so we 

 shall be unavoidably led to allude to some classes of phenomena which 

 the beginner must not yet expect fully to comprehend. 



It was for many years a received opinion, that the formation of entire 

 families of rocks, such as the plutonic and those crystalline schists spoken 

 of in the first chapter as metamorphic, began and ended before any mem- 

 bers of the aqueous and volcanic orders were produced ; and although 

 this idea has long been modified, and is nearly exploded, it will be neces- 

 sary to give some account of the ancient doctrine, in order that beginners 

 may understand whence many prevailing opinions, and some part of the 

 nomenclature of geology, still partially in use, was derived. 



About the middle of the last century, Lehman, a German miner, pro- 

 posed to divide rocks into three classes, the first and oldest to be called 

 primitive, comprising the hypogene, or plutonic and metamorphic rocks ; 

 the next to be termed secondaiy, comprehending the aqueous or fossilif- 

 erous strata ; and the remainder, or third class, corresponding to our 

 alluvium, ancient and modern, which he referred to " local floods, and 

 the deluge of Noah." In the primitive class, he said, such as granite 

 and gneiss, there are no organic remains, nor any signs of materials de- 

 rived from the ruins of pre-existing rocks. Their origin, therefore, may 

 have been purely chemical, antecedent to the creation of living beings, 

 and probably coeval with the birth of the world itself. The secondary 

 formations, on the contrary, which often contain sand, pebbles, and or- 

 ganic remains, must have been mechanical deposits, produced after the 



