Review of Recent Geological Literature. 133 
roka volcanoes in Miocene, while the great fissure eruptions just men- 
tioned took place in the Pliocene. 
A map accompanies this sketch showing the surface distribution of 
these three parts. 1st the andesyte, 2nd the rhyolite and 3rd the basalt. 
The volume of igneous rock emptied to form the andesytes of the Absa- 
roka range and the surroundiog areas is estimated, within the bounds 
of reason, at 4000 cubic miles. The floods of rhyolite that escaped at 
the time of the great tissure eruptions now cover an area 50 miles from 
east to west, and 90 miles from north to south, and it continues further, 
but is covered by later basalt. The volume is much more than 400 cu- 
bic miles. The basalt which issued later from vents or fissures further 
west, must have a volume of at least 700 cubic miles. These vast ex- 
trusions manifest fixed variations in acidity, establishing, in the view of 
Prof. Iddings, the general law that first come eruptions of medium acid- 
ity (andesyte) followed by very acid (rhyolite) and then by very basic 
(basalt), the latter two being closely associated and representing the 
complementary extremes of differentiation. He concludes : " It was 
upon evidence of this order that I ventured to enunciate the principle 
that in a region of eruptive activity the succession of eruptions com- 
mences in general with magmas representing a mean composition and 
ends with those of extreme composition." n. h. w. 
On the soiitJiern Devonian formation. H.S.Williams. (Amer. Jour. 
Sci., f4) III, .393-403.) In this Prof. Williams has attempted to restore 
some of the features of the Devonian geography of the eastern part of 
this continent. Every such contribution is welcome to the student of 
paleozoic geology, and Prof. Williams has made so extensive and contin- 
ued a study of the subject, especially in its palseontologic aspects that 
whatever he puts forth deserves careful consideration. 
In grappling with these problems the author frankly confesses the 
immense difficulty which some of them present. We owe to him -viery 
largely the indications of what seems to be at least a probable opinion 
that a radical change oc;curred in the mid-Devonian life erf the Appala- 
chian region by the immigration of a fauna hitherto existing in the 
northwest. But as with others it is not easy for Prof. Williams to indi- 
cate where lay the barrier whose removal allowed them passage. That 
the Wisconsin peninsula extended far enough southward to bar the 
way seems very difficult of admission and yet no more j)lausible theory 
can be at present put forward. 
But the problem of the great black shale is the one to which Prof. 
Williams has chiefly turned his attention in the tract before us, and of 
this he says frankly: "I have been unable to arrive at any satisfactory 
explanation of it." 
A recent study of this shale in Virginia, Tennessee and Kentucky, 
where in some cases it is of unusual thickness, added nothing to the 
scanty fauna. But Prof. Williams has been led by this work to venture 
the opinion that the material of the black shale was not derived from 
the east but from the western side of the channel or gulf of Appalachia. 
