1891.] Mineralogy and Petrography. 1003 
diabase, and the other an olivine-hypersthene-diabase. The latter has 
been found only in one place,—viz., two miles north of Rapidan Station 
on the Virginia Midland R. R. The hypersthene-diabase occurs not 
only in Various places in Virginia, but it has also been found- north. 
It is slightly ophitic, inclining somewhat to the porphyritic. Its feld- 
spar is a basic labradorite of the formula Ab,An,. The pyroxenes are 
a slightly pleochroic, polysynthetically twinned diallage, and a 
strongly pleochroic hypersthene. The former contains numerous 
microlitic inclusions, while the latter is free from them. Quartz, apa- 
tite, and green hornblende are accessories. In the olivine rock the 
olivine is largely idiomorphic, and is in large grains. The other con- 
stituents are the same as those of the hypersthene-diabase. The paper 
contains an excellent series of analyses of the rocks and their most 
important components. Barrois* has given us a masterly discussion of 
the diabases and diabase-porphyrites of Silurian age, occurring agdykes 
and flows in Menez-Hom, Finistère, France. The diabases he divide 
according to structure into granular and ophitic types. Among the 
former are olivine-bearing varieties, sometimes containing hypersthene, 
and olivine-free kinds, containing orthoclase, quartz, and occasionally 
porphyritic augite. The porphyrites are divided into andesitic varie- 
ties in which the feldspar-microlites are older than the augite, and into 
variolitic kinds with feldspar younger than augite. A large part of the 
rocks of the region studied occur in the form of tuffs, in which the 
cementing material is shale, limestone, or sandstone, and the fragments 
are sometimes large enough to be called bombs. Schalstein is also 
common. The contact effects noticed in the eruptives are insignifi- 
cant in amount. The schists in contact with the bedded diabases are 
spilosites, containing nodules of chlorite. The most interesting con- 
tact effects are those noticed in the case of nodules originally consisting 
of pyroxene and quartz. Under the influence of sea-water made hot 
by submarine ejections, these nodules have become zonal, in which the 
two zones differ principally in the size of their constituents, as 
both contain pyrite, albite, quartz, sphene, and limonite. In 
conclusion, the author makes some general remarks on the study o¢ 
ancient volcanoes, and gives quite a good résumé of the work done in 
this direction. A dyke of basic rock on Stop Island, in Rainy Lake, 


Canada, consists of diabase-porphyrite with an almost aphanitic texture 
on its contact. It contains occasional rounded masses of augite. Four 
feet from the contact it is a diabase, with the ophitic texture, and at 
fifteen feet from the contact it is also a diabase. Here, however, a 
t Bull. Serv. Carte Geol. d. Fr., No- 7, 1890. 


