1890]. Mineralogy and Petrography. 1187 
a tributary of the upper Lahn, which on its upper surface bears flowage 
marks like those in modern lavas. A section from this portion of the 
rock-mass shows a glassy base, dotted with grains of magnetite and 
mottled with irregularly shaped, doubly refracting areas, without fur- 
ther definite characteristics. Beneath the surface the mottlings resolve 
themselves into fibrous lath-shaped feldspar crystals, arranged in fluidal 
lines and extinguishing with a somewhat undulous extinction. The 
mass between these appears to be homogeneous, but in polarized light 
it is found to possess aggregate polarization. Ata distance of twenty 
centimetres from its surface the rock consists of well-defined feldspar 
laths and little grains of augite and altered ilmenite. At 60-100 cm. 
from this surface it isa typical diabase. The interesting features con- 
nected with the occurrence are: r, the undoubted former existence of 
the rock magma as a fluid upon the surface, as indicated by the struc- 
ture of its upper portion and the flowage lines marked by the feldspar 
crystals; 2, the existence of the typical diabase texture (hypidiomor- 
phic-granular) of the rock at but a little distance beneath its upper 
surface. The lack of crystals in its upper portion shows that crystalli- 
zation began only after this part of the magma had come to rest. It 
was during this period of rest that all the crystallization took place ; 
hence, the author concludes, rest and gradual cooling are the condi- 
tions necessary to the assumption of the hypidiomorphic structure. 
Continuing the subject further, the same writer describes an occurrence 
of diabase in a flow at Homertshausen, in Nassau. At some distance 
beneath its upper surface, where crystallization went on gradually, the 
rock is a normal diabase with an ophitic structure, and possessing no 
olivine. Nearer the surface it is rich in corroded olivine, poor in 
augite, and it contains radially divergent feldspar crystals cemented by 
glass. Nearer to the periphery it is composed of glass, holding crys- 
tals of augite and varioles of the composition and structure of diabase 
(concretions). On the periphery it is a glass with globulites, globosphe- 
rites, etc. The minute structure of each of these phases is described 
in great detail, as is also the effect of the solution of limestone inclu- 
sions upon the diabase material surrounding them. In the third? 
division of his paper the author announces that Rosenbusch has 
decided to place the diabases with the effusive rocks, and then dis- 
cusses their position in the scheme with respect to other basic effusives. 
He shows that there is no definiteness in the distinctions between 
augite-porphyrite, melaphyre, basalt, and diabase. Every definition 
that is proposed for any one of these rocks breaks down when exam- 
s Ib., p. 523. 
