182 The American Geologist. August, 1897 
Extrusive and intrusive igneous rocks as products of magmatic dif- 
fereniiation. *J. P. Iddings. (Quart. Jour. Geol. Soc, vol. lit (1896) 
pp. 60G-617). In a recent important publication (Eruptionsfolge der triad- 
ischen Eruptivgesteine bei Predazzo in Siidtyrol) Prof. W. C. Brogger 
expressed dissent from some of the inferences of Prof. Iddings from his 
study of the rocks of Electric peak and Sepulchre mountain in the Yel- 
lowstone park, and in this paper Prof. Iddings explains more fully the 
relations of those eruptions to the whole series of which they are but a 
part. Brogger's chief criticism, or difference of opinion, was based on 
the idea that the extruded rocks examined by Iddings were of a limited 
amount and extent, and that the primary differentiations of the deep- 
seated magma should be determined by a study of deep-seated and not 
of effusive rocks. Iddings sketches the volcanic history of the great 
floods of Montana, Idaho and Wyoming, as follows : 
At the end of Cretaceous time there were great orographic movements 
by which the region was profoundly faulted and dislocated. Volcanic 
action began at this time, and fragments of igneous rocks are included 
in the sandstones immediately overlying the coal-bearing Laramie, which 
witnessed the close of the long period of quiet which had prevailed dur- 
ing Paleozoic and Mesozoic times in that region. After this upheaval 
there was extensive surface denudation whereby the sedimentary strata 
were entirely removed from off the underlying crystalline schists. Into 
these disturbed strata the igneous material was forced, and upon the 
uneven surface of the country it was spread out, covering crystalline 
schists in one place, and upturned sandstones and limestones in another. 
Id most cases the character of the eruptions was extremely violent. At 
first they were largely explosive, sliattering the surface rocks, whether 
gneiss or limestone, and scattering them broadcast to form the first 
layer of tuff-breccia, or to Vje ultimately mingled with the fragments of 
lava. The explosive character prevailed until a great accumulation of 
tuff-breccia formed a chain of lofty volcanoes comparable with those 
of the Andes in size as well as in the nature of their material. The later 
eruptions from these volcanoes were quieter outflows of lava, which 
probably took place after position of the volcanic conduits had become 
more stationary. Erosion having carried away the upper parts of these 
great cones, the remaining portions are almost equally made up of brec- 
cia, in places 4000 feet thick. 
The last of the great eruptions were equally violent, though of a dif- 
ferent kind. They were gigantic flssure-eruptions that flooded the re- 
gion west of the chain of denuded volcanoes with massive streams of 
lava that I'ose high up on the flanks of the sun-ounding mountains, and 
then flowed toward the southwest, leaving when cooled and after erosion 
had somewhat reduced the surface of the stream, a vast sheet of lava 
at least 1000 feet thick in most places, and over 2000 feet thick in some 
parts. This was followed by other outflows from fissures that flooded 
the region for hundreds of miles to the southwest and west and closed 
the period of activity. Within the Yellowstone park the earliest brec- 
cias were accumulated in Eocene time, and the great bulk of the Absa- 
