412 HE JOURNAL OLNGEOBOGNs 
this article, no eruptions are known to have occurred since early 
Pleistocene time. 
The succession of the Tertiary volcanic rocks is then as fol- 
lows: 
Acip - - - - - - - Rhyolite—massive and fragmental. 
Basic- - - - - - - Older basalt —always(?) massive. 
INTERMEDIATE- - - - Hornblende-pyroxene-andesite — chiefly tuff 
and breccia. 
INTERMEDIATE TO ACID - Fine-grained pyroxene-andesites — massive. 
BASIC - - - - - -  Doleritic basalts — massive. 
Basic - - - - - - - Other basalts — massive. 
This succession does not accord with the law of Richthofen 
nor with Iddings’ law, but it should be noted that the record 
here given is an imperfect one. 
According to Dr. James Blake,’ there is a variety of volcanic 
rocks in the Puebla Mountains in Northern Nevada forming suc- 
cessive flows, the relative age of which is very clear, as the 
series is finely exposed. Dr. Blake made some chemical deter- 
minations of the igneous rocks, but they do not appear to have 
been studied microscopically. Indeed, at that time (1873) the 
microscope was in little use in rock investigation. His conclu- 
sion as to the succession is that it does not accord with Richt- 
hofen’s law, but the series he describes obviously needs further 
investigation. 
Iddings’ law? is perhaps the latest that has been formulated 
with great care and after a consideration of the succession in 
many parts of the world, with a large amount of original 
research based on extensive field study and collections at vari- 
ous volcanic centers in the West. It is as follows: 
The variation in the composition of igneous rocks, which constitute a 
series of eruptions at any volcanic center, is the result of the chemical differ- 
entiation of some intermediate magma. The composition of the inter- 
mediate magma may be different in different centers of eruption and in dif- 
ferent regions, and it will be shown subsequently that the intermediate 
magma of any particular center may itself be the result of a differentiation 
of a more ancient magma or of a primary uniform magma, if such a thing 
can be shown to have existed. 
*Proc. Cal. Acad. Sci., Vol. V., pp. 210-214. 
2 Bull. Phil. Soc., Washington, Vol. XII, p. 151. 
