152 



LYELL'S ELEMENTS OF GEOLOGY. 



Age of Rocks Lehman's and Werner's Classification of Rocks. 



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

 of whole classes of rocks, such as the plutonic and metamorphic, 

 began and ended before any members of the aqueous and volca- 

 nic orders were produced ; and although this idea has long been 

 modified, and is nearly exploded, it will be necessary to give 

 some account of the ancient doctrine, in order that beginners 

 may understand whence part of the nomenclature of geology still 

 partially in use was derived. 



About the middle of the last century, Lehman, a German mi- 

 ner, proposed to divide rocks into three classes, the first and old- 

 est to be called primitive, comprising the plutonic and metamor- 

 phic rocks ; the next to be termed secondary, comprehending the 

 aqueous or fossiliferous strata ; and the remainder or third class, 

 the supposed effect of " local floods, and the deluge of Noah," 

 corresponding to our alluvium, ancient and mordern. In the 

 primitive class, he said, such as granite and gneiss, there are no 

 organic remains, nor any signs of materials derived 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 se- 

 condary formations, on the contrary, which often contain sand, 

 pebbles, and organic remains, must have been mechanical de- 

 posits, produced afler the planet had become the habitation of 

 animals and plants- This bold generalization, although antici- 

 pated 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 dis- 

 criminating the mineralogical 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 and clay-slate, and 

 yet exhibiting here and there signs of a mechanical origin and 

 organic remains. For this group, therefore, forming a passage 

 between Lehman's primitive and secondary rocks, the name of 

 transition was proposed. They consisted principally of clay- 

 slate and an argillaceous sandstone, called greywacke 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 the newer and fossilife- 

 rous rocks were horizontal. To these latter, therefore, he gave 



