350 WHITMAN CROSS 
work of a German, for in it he breaks away from the influence of 
the Wernerian school. This was doubtless to some extent the 
result of a visit to Paris, but while building upon the character- 
istics of rocks he did not give to mineral composition the com- 
pletely dominating place which it held in the French systems. 
Von Leonhard rejects the biological framework of classes, 
orders, genera, and species as quite inapplicable to rocks, since 
they lack individuality. 
Alexandre Brongniart, 1827.—The Classification et characteres 
mincralogiques des roches homogénes et hétérogenes, by Alexandre 
Brongniart, published in 1827, marks the next important step in 
the development of petrography. This admirable little book of 
only 144 pages contrasts markedly with other voluminous works 
of its period. It is concise in statement, and presents a clearly 
conceived and logically worked out system. 
In the introduction we find minerals, rocks, and geological 
terranes distinguished and defined. Minerals are species or varie- 
ties determined by the laws of mineralogy; rocks are the same 
substances considered in their masses and as entering into the 
structure of the globe; ¢erranes are assemblages of several rocks, 
considered as having been formed at about the same epoch. 
Brongniart pointed out that there were two ways of looking at 
rocks —as to their composition, and as to their occurrence. He 
held that geognosy was not strictly a classification of rocks; 
that to arrange rocks by occurrence and describe them in that 
order involved many digressions from the discussion of relation- 
ships; that rocks formed at different times must then be referred 
to as many times as they occurred; that the same name would 
be given to different substances formed at the same time; and, 
finally, that classification by occurrence involved the use of 
hypotheses where knowledge was lacking. All these troubles 
should be avoided, in his opinion, by classifying and naming 
rocks upon mineralogical composition and independently of 
occurrence. The treatment from the latter standpoint could 
then follow naturally. 
The system of Brongniart is a mineralogical classification 
