PETROGRAPHY. 325 



in which the chief mineral constituents lie imbedded in a 

 "matrix'' or ground-mass, e.g., gneiss, schist, phyllite, porphyry, 

 trachyte, obsidian, lava ; 3, the " aggregated " or fragmental 

 varieties, which take origin by mechanical means, and whose 

 ingredients are cemented together by subsequent infilling of 

 material, e.g., psammites (sandstone, greywacke), pudding- 

 stones, and breccias. 



Brongniart, as well as his predecessors Hauy and Cordier, 

 confined themselves exclusively to the mineralogical com- 

 position and structure of the rocks, without respect to their 

 mode of occurrence, their age, or their origin. While this 

 method of treatment proved undoubtedly beneficial to the 

 development of systematic petrography, it endangered the 

 connection between geology and petrography, and in this 

 respect the direction initiated by the French petrographers 

 must be regarded as retrograde in comparison with the 

 Wernerian School. 



The best and most complete work of that time on petrology 

 was Leonhard's 1 Charakteristik der Felsarten (1823-24). In 

 this work likewise the mineralogical standpoint predominates, 

 but the Wernerian influence is apparent in the frequent 

 digressions which give information regarding the occurrence 

 of the different kinds of rock in the field and their mode of 

 origin. Leonhard distinguished four sub-divisions of rocks : 

 T, rocks composed of unlike elements; 2, rocks apparently 

 uniform ; 3, derivative or fragmental rocks ; 4, friable and in- 

 coherent rocks. As all Leonhard's distinctions were founded 

 on macroscopic examination of the rocks, the group of the 

 "apparently uniform" rocks is quite artificial, and the limits 

 of the others are unsatisfactory. 



Cordier 2 had suggested in 1815, according to the precedent 



1 Carl Casar von Leonhard, born 1779 at Rumpenheim near Hanau, 

 studied in Marburg and Gottingen ; in 1800 entered into the Hessian 

 Government Service; in 1810 was appointed Councillor of the Exchequer 

 in Chur Hesse, and afterwards Director of Domains; in 1816 accepted a 

 call to the Munich Academy, but left Munich in 1818 to be Professor of 

 Mineralogy and Geognosy at the University of Heidelberg, where he died 

 on the 23rd January 1862. 



2 Pierre Louis Antoine Cordier, born 1 777 i n Abbeville, began life as 

 a mining engineer in 1797 ; took part under Dolomieu in the Egyptian 

 Expedition ; in 1819, succeeded Faujas de Saint-Fond as Professor of 

 Geology at the Botanical Garden, and at the Restoration of the Empire 

 was made a peer of France. He died in 1862. 



