SS REMATT EC PEDROGRA PLT Ya 369 
ciples especially named as used in this way are, crystalline text- 
ure, lack of stratification, predominance of silicates, etc. To 
von Richthofen it appeared that the exact mineral composition 
of rocks asa basis for their classification had become possible 
after the investigations of Gustav Rose on the feldspars, and that 
by this means petrography had been brought out of a chaotic 
state. While acknowledging, therefore, the high value of min- 
eral composition as a basis for classification of igneous rocks, von 
Richthofen considered that its exclusive application had grouped 
rocks geologically far separated, and distinguished rocks geo- 
logically closely connected, which seemed to him a funda- 
mental error. 
The ‘natural system” of igneous rocks proposed by this 
author was based upon the Bunsen law of two fundamental 
magmas, and upon what he considered to be demonstrated facts 
of a broad correlation between developed texture and age of 
igneous masses, and of an order of succession of magmas in the 
history of the earth. By using ‘‘eruptive”’ as a collectivé term 
for all rocks under discussion, he implied the adoption of geologi- 
cal mode of origin as the first principle in classification. Von 
Richthofen remarked that Bunsen’s law might have to be revised, 
‘but no change of its principles may ever be expected, as an 
overwhelming amount of evidence has accumulated in support 
of its essential tenor.” Texture is used by von Richthofen as 
the second principle in system to produce three classes of erup- 
tive rocks: granite, porphyritic, and volcanic. He says that the 
conclusion appears to him to be justified ‘‘that the three great 
classes of eruptive rocks are geologically separated and represent 
three successive and distinct phases of the manifestation of sub- 
terranean agencies.” The granites of the Sierra Nevada, with 
which he had personally become familiar, forced von Richthofen 
to admit that in some parts of the world the ancient granitic and 
porphyritic eras were succeeded by later eras of the same rocks, 
within the Mesozoic, but he believed that the volcanic era began 
with the Tertiary, bothin Europe and America. The old lavas 
of the British Isles, known since the early decades of the 
century, were either disregarded or overlooked by him. 
