SYSTEMATIC PETROGRAPHY 349 
selves under the Homogeneous rocks, with the heading, ‘‘ Rocks 
(apparently homogeneous) which are not to be regarded as 
minerals” (‘‘Nicht als Glieder oryktognostischer Gattungen zu 
betrachtende—scheinbar gleichartige—Gesteine’”’). Here we 
find trachyte, aphanite, serpentine, basalt, pitchstone, obsidian, 
and various schists. Von Leonhard realized more or less clearly 
that most of these rocks would some day be found to be com- 
plex in character, and he therefore appreciated that his treat- 
ment of them was a makeshift. Twelve years later, in his 
Lehrbuch der Geognosie und Geologie, he raised the apparently 
homogeneous rocks to an equal rank with the heterogeneous and 
homogeneous divisions. 
Von Leonhard was unquestionably the foremost petrographer 
of his day, sharing with Alexandre Brongniart the honor of first 
placing the classification of rocks upon a firm basis as a system- 
atic science. He had been called to Heidelberg but a few years 
before this work was issued, finding there perhaps the largest 
collection of rocks then in existence. He had also visited Paris 
and studied the great collection of the Museum of Natural His- 
tory as arranged by Hatiy. Von Leonhard also belonged to the 
group of German geologists who, with Smith and Macculloch 
in England, and a few elsewhere, were engaged in placing 
stratigraphic geology upon a sure foundation. He did not him- 
self develop the distinction, clearly foreshadowed in his work, 
between the geological formation and the rock. 
A critical examination of the system contained in the Char- 
akteristik shows that von Leonhard actually applied as his first 
criterion the distinction between massive and clastic rocks, as we 
should express it, though he may not have realized it. Next he 
applied the idea of homogeneity and its opposite. But the not- 
able feature is the prominence given to structure and the close 
approximation of some of his definitions to ones in current use. 
It was the first time structure was assigned so prominent a role 
in a system of rocks. Mineral composition followed structure, 
and was applied quite reasonably. 
The system of von Leonhard is specially noteworthy as the 
