1004 . The American Naturalist. [November, 
portion of the augite is in idiomorphic crystals and in polysomatic 
grains, while another portion is in allotriomorphic masses between the 
feldspars. In the middle of the dyke the texture is gabbroitic, while 
hornblende replaces the augite. Quartz is also a prominent constitu- 
ent of the center of the dyke, whereas it is only sparingly present at 
fifteen feet from the contact, and is entirely absent at the contact. Dr. 
Lawson 5 calls attention to these facts, and states that in the single geo- 
logical unit represented by the mass of the dyke we must distinguish 
between three distinct rock types if we make use of present methods of 
nomenclature. He further thinks that the phenomena indicate that 
rate of cooling, rather than pressure, is the principal cause determining 
the textural condition assured by a solidifying magma. Other dykes 
exhibiting similiar pecularities are described from other localities in 
the Rainy Lake region. All show larger percentages of SiO, in their 
middle portions than are shown near their contacts,—a fact ascribed to 
the separation of basic augite, magnetite, etc., in those portions that 
cooled rapidly. The larger part of the area of the Dippauer Gebirge 
in Northwestern Bohemia is covered by basalts, occurring in flows, 
dykes, and bosses, with their tufas and conglomerates. These are cut 
by phonolites and andesites, in the former of which are large grains of 
perofskite, in some cases showing parallel striations in parallel light. 
The basalts occur in all varieties, according to Clements. The cen- 

tral portions of the hills are composed principally of leucite and neph- 
eline basalts and the closely related rocks, nephelinites, leucitites, 
nepheline and leucite tephrites, and leucite-basanites. On the peri- 
pheries of the mountains are feldspathic basalts, limburgites, and augites. 
Among the most interesting features of the several rocks noticed are 
corroded biotite plates, surrounded by rims formed of secondary crys- 
tals of the same mineral, in the nepheline basalts; pseudomorphs of 
phillipsite after olivine in the leucitites ; augite crystals with second- 
ary twinning lamellae produced by pressure in the nephelinites and 
feldspathic basalts ; zonal augites in the leucitites, with an extinction 
varying gradually from the center to the periphery, and others with the 
hour-glass structure and an outer zone containing colorless microlites 
with their long axes lying parallel to the bounding walls of the crys- 
tals; and sanidine inclusions surrounded by augite crystals in leucite 
basalt. Leucite is more abundant in close proximity to an orthoclase 
inclusion in the last-mentioned rock than elsewhere in the rock-mass. Of 
the feldspathic basalts it was found that the youngest is most acid.—— 
5 Amer. Geol., VIL., 1891, p. 153. 
_ 8 Jahrb. d. Kais-Kön. geol. Reichsanst, 1890, XL., p. 317. 

