No. 423.] NOTES: AND LITERATURE. 257. 
Basic Rocks in Maryland. — A series of acid and basic rocks 
intrude the gneisses of northeastern Maryland, and these in turn are 
cut by extensive dykes of granite and pegmatite. The principal 
types are biotite granite, diorite, tonalite, norite, hypersthene-gabbro, 
pyroxenite, peridotite, and serpentine. The order of eruption seems 
to have been norite and gabbro, diorite and granite. The last two 
rocks are connected by gradation phases, and this is thought to be 
evidence that their difference in age is not great. The peridotites 
and pyroxenites are mainly younger than the norites and gabbros, 
but some of the pyroxenites are apparently peripheral phases of the 
norite, 
The succession taken as a dol t is supposed to * furnish an exam- 
ple of the occurrence of several rock types which represent the facies 
of a single magma and unite to form a geological unit." 
The serpentines were derived partly from the peridotites and 
partly from the pyroxenites. In the latter change the hypersthene 
and diallage first alter into fibrous hornblende, and this later passes 
into the serpentine. The area is very similar in its essential features 
to the Delaware area of gabbros described by Chester and the Balti- 
more area described by Williams. The distinguishing features of 
the present area are (1) the abundance of diorite, (2) the eompara- 
.tive rarity of gabbro-diorite, and (3) the great abundance of t 
tite and granite dykes.! 
Rock Structure. — The study of a series of andesitic and rhyolitic 
rocks that occur as extrusives in the Great Basin region suggests to 
Spurr? that the differences in structure of igneous masses may be 
brought about by slight changes in conditions under which crystalli- 
zation took place. Among the differences which affect structure may 
be mentioned slight changes in the rate of cooling. Textural varia- 
tions are less common in acid than in basic extrusives because in the 
latter the viscosity varies more rapidly with rapid cooling than in 
the former. The more important structures are not characteristic of 
particular rocks, but are the functions of the relation between viscos- 
ity and cooling rate. Structure therefore cannot, according to the 
author, be made the principal element in rock classification. 
1 Leonard, A. G. Amer. Geol. (1901), p- 135. 
2 Journ. Geol., vol. ix (1901), p. $86. ` 
