No. 413.] REVIEWS. OF RECENT LITERATURE. 2 
425 
Occurrences of Differentiated Magmas. — As the result of a rapid 
survey of the Magnet Cove, Arkansas, igneous area, Washington! 
concludes that the complex described by Williams represents an 
excellent though peculiar example of a highly differentiated magma, 
probably in the form of a laccolite, and not a series of independent 
intrusions. The rock types present in the area form a regularly 
graded series, ranging from foyaite, through leucite-porphyry, shon- 
kinite-porphyry, ijolite, and biotite-ijolite to jacupirangite. The 
distribution of the rocks is, however, abnormal, since the basic 
varieties are near the center of the supposed laccolite, and the acidic 
varieties in its periphery. The author explains this abnormality by 
supposing the magma to be a solution in which the solvent was in 
great excess. This solidified first on the outside, leaving a more 
concentrated, and consequently a more basic solution within. The 
solvent continued to separate by crystallization as a more and more 
basic rock, as the cooling continued inward, until, finally, at the 
center the most concentrated and most basic material solidified as 
jacupirangite. | 
purr? has compared the order of succession of the intrusions 
in the Great Basin and has found it to be in general the same 
throughout the district, although at any given place certain members 
of the series may be lacking. This succession as worked out is 
as follows: acid rocks (rhyolites), siliceous intermediate types 
(andesites), acid rocks, associated with basalts, basic intermediate 
types (more basic andesites and aleutites), basic rocks (basalts), 
with associated rhyolites. The Great Basin appears to have been 
underlaid by a single body of molten magma which supplied at differ- 
ent times lavas of similar composition to all the different parts of 
the overlying surface. Since this succession is different from any 
described from other regions, the author suggests that the first 
rhyolite is the end member of a series of differentiates, and that the 
andesites are the beginnings of two distinct cycles of differentiation. 
He further suggests that during the first completely recorded cycle 
beginning with the earlier andesite the siliceous differentiates of this 
magma were erupted in preference to the basic differentiates, while 
in the second cycle the basic members were the predominant extrava- 
sations. It must be borne in mind that the processes of differentia- 
tion are independent of the causes that produce expulsion of lavas, 
and hence the records of the differentiation, as observed on the 
1 Bull. Geol. Soc. Amer., vol. xi, p. 389. 
2 Journ. of Geol., vol. viii, p. 621. 
