1892,] Mineralogy and Petrography. 947 
grains, the former in dodecahedral crystals that have in many instances 
been shattered by pressure, and the latter in bent plates with the usual 
features of cyanite. Biotite encircles both of these minerals, notably 
the garnet, as a sort of zone. The groundmass in which these compo- 
nents lie is an aggregate of oligoclase, orthoclase and quartz, some- 
times the monoclinic and at other times the triclinic feldspar predomi- 
nating. Rutile is present in the rock as inclusions in the garnet, the 
cyanite and the biotite, as an alteration product of the mica, and as 
crystals in the quartz-feldspar aggregate. Muscovite, ilmenite, zircon 
and leucoxene are also present in small quantities. 
Tufaceous Slates from Wales.—Among the sedimentary 
roofing slates of North Wales Hutchings‘ finds some that appear to be 
composed principally of andesitic and rhyolitie ash, consisting of 
fragments of lapilli, of feldspar and of sedimentary rocks imbedded 
in a paste of chlorite, small rods of sericite and minute grains of gar- 
net, besides a little quartz and calcite. The most essential differences 
between these slates and those of sedimentary origin are with respect 
to their titanium constituents; in the ashes sphene and anatase being 
the most important, and in the true slates the so-called “ slate-needles.” 
These latter are thought by the author to occur only as decomposition 
products of biotite, and to be limited in their occurrence to water 
deposited fragmentals. The feldspar in the rocks under discussion are 
changed to white mica, chlorite and calcite. Secondary orthoclase 
and plagioclase often coat tiny cavities in the -rock. 
Alteration Products of Diabase from Friedensdorf.—The 
clefts in the diabase of Friedensdorf, near Marburg, are covered with 
little crystals of albite, analcite, natrolite, prehnite and calcite, all of 
which minerals occur also in the body of the rock. According to 
Brauns’ they are decomposition products of the diabase plagioclase, 
and are due to the action of water containing carbon-dioxide upon 
this feldspar. Microscopic sections show the original plagioclase sur- 
rounded by fresh albite and filled with little nests of the other second- 
ary substances mentioned. The process of the alteration is outlined 
by the author, who also shows the chemical relations existing between 
the new substances and the material from which they were derived. 
The diabase originally contained in addition to the plagioclase, both 
monoclinic and orthorhombic pyroxenes, olivine and titanic magnetite. 
*Geol. Magazine, 3, ix, 1892, pp. 145-335. 
5Neues. Jahrb. f. Min., etc., 1892, ii, p. 1. 
