THE MAKING OF THE GEOLOGICAL TIME-SCALE. 1/ 



Floctsgcbirgc {dd'd) were deposited the loose-lying gravels 

 and soils of the valleys, c, of the rivers (alluvial) and of their 

 flood-plains (diluvial).* 



Lehmann's classification, in so far as it goes, expressed 

 established facts of nature. There are Primitive, Secondary, 

 Tertiary, and Quaternary formations, but the theory that 

 they may be defined and determined by physical structure 

 and present relative position is only approximately true. 

 All crystalline rocks are not primitive, all the secondary rocks 

 are not merely consolidated fragments of primitive rocks. 

 Some of them are fully metamorphosed. All Tertiary rocks 

 are not unconsolidated, as the Tertiaries of California illus- 

 trate, and we now know that altitude above the sea, or rela- 

 tive position of the various formations, is by no means 

 uniform and forms no criterion for their determination. 



Werner's Classification of Rocks by their Mineral Characters. 

 — The next important advance in the classification of rocks 

 was started by Werner and his pupils. It was a classification 

 based upon the mineral constitution of the rocks. As the 

 study of geology advanced Lehmann's classification was .found 

 difficult to apply with precision, and it was found to be un- 

 natural in that rocks of apparently similar kind were dis- 

 sociated, while rocks of unlike character were brought into 

 the same class. And the mineral character and composition 

 of rocks was found to be an accurate means of defining them. 

 As the mineral characters became clearly understood, the 

 rock masses received their names from the chief minerals in 

 them, and finally the mineral nomenclature entirely super- 

 seded the nomenclature of Lehmann, and a second classifica- 

 tion arose in which the theory of the original order of forma- 

 tion of the rocks gave place to the actual sequence of mineral 

 aggregates, one after another, in examined sections of the 

 earth's crust. In this study of minerals Werner was a con- 

 spicuous leader, and the classifications at the beginning of 

 the present century were mainly his or adaptations of them. 



* W. D. Conybeare and William Phillips. " Outline of the Geology of England 

 and Wales, with an introductory compendium r f the general principles of that 

 science, and comparative views of the structure of foreign countries," Part I. 

 p. xix. 



