36 CLASSIFICATION OF KOCKS. [Ch. VIII. 



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 secondary, 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 rains 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 

 planet had become the habitation of animals and plants. This bold 

 generalization, although anticipated in some measure by Steno, a century 

 before, in Italy, formed at the time an important step in the progress of 

 geology, and sketched out correctly some of the leading divisions into 

 which rocks may be separated. About half a century later, Werner, so 

 justly celebrated for his improved methods of discriminating the minera- 

 logical characters of rocks, attempted to improve Lehman's classification, 

 and with this view intercalated a class, called by him " the transition 

 formations," between the primitive and secondary. Between these last 

 he had discovered, in northern Germany, a series of strata, which in their 

 mineral peculiarities were of an intermediate character, partaking in 

 some degree of the crystalline nature of micaceous schist and clay-slate, 

 and yet exhibiting here and there signs of a mechanical origin and or- 

 ganic remains. For this group, therefore, forming a passage between 

 Lehman's primitive and secondary rocks, the name of ubergang or transi- 

 tion was proposed. They consisted principally of clay-slate and an ar- 

 gillaceous sandstone, called grauwacke, and partly of calcareous beds. 

 It happened in the district which Werner first investigated, that both the 

 primitive and transition strata were highly inclined, while the beds of 



