360 WHITMAN CROSS 
I. Eruptive rocks, probably all originating through consolidation from a 
molten condition. 
1. Poor in silica, or Basic. 
a. Volcanic. 
6. Plutonic. 
2. Highly siliceous, or Acid. 
a. Volcanic. 
6. Plutonic. 
II. Metamorphic crystalline schists, probably all derived from sediments, 
yet resembling eruptive rocks in mineral composition. 
III. Sedimentary rocks. 
1. Argillaceous. 
2. Calcareous. 
3. Siliceous. 
4. Tuffs. 
To these three clearly defined groups was added an appen- 
dix of rare rocks or those of problematic origin. Here were 
included many quartzose rocks, coals, iron ores, serpentine, etc. 
The eruptive rocks were considered as derived from molten 
magmas; the broad chemical distinction was made with Bun- 
sen’s law in mind, but not directly as its expression ; the volcanic 
and plutonic rocks were separated purely on the basis of occur- 
rence; the further subdivision was by general mineral composi- 
tion, and texture was used as subordinate to it. It is worthy of 
special note that von Cotta expressly pointed out that geologi- 
cal age was not involved in the distinction between the volcanic 
rocks and the plutonics. He considered that the older volcanics 
had been largely removed by erosion, and that the younger plu- 
tonics had not yet been laid bare. 
Ferdinand Senft, 1857.—In 1857 appeared an elaborate work 
by Ferdinand Senft, with the title (translated) Classification and 
Description of Rocks, Founded upon theiy Mineral Constitution, 
Chemical Composition, and Structure.* This treatise, which received 
the Demidoff prize (St. Petersburg), is mainly notable for its 
elaborate attempt to apply chemical factors in details of classt- 
fication. 
1 Classification und Beschreibung der Felsarten, gegriindet auf ihre mineralo- 
gische Beschaffenheit, ihre chemische Zusammensetzung und ihre Structur,” Breslau, 
1857. 
