364 The American Naturalist. [April,, 
which was reported by Lawson’ as representing an old basement lying 
unconformable beneath the gabbro. 
The Geology of Dartmoor, England.—McMahon‘* gives a few 
brief descriptive notes on some trachytes, felsites, mica-diorites, dole- 
rites, tuffs and hornblende-schists from the western flank of Dartmoor. 
The trachytes and felsites are more or less altered, and the tuffs always 
very much so. The tuffs contain fragments of several kinds of lavas 
and of altered sedimentary rocks. The cementing material is “ like the 
microgranular base of some rhyolites and porphyries.” The most 
interesting rocks are the hornblende schists, which are thought by the 
author to be altered basic tuffs. They are marked by a fine grained 
parallelism of their constituents, producing a structure which the 
author designates the “corduroy structure.” The rocks consist of 
angite, secondary hornblende and feldspar, the first two of which are 
often well crystallized. Their alteration is thought to be due to the 
intrusion of the tuffs by the great mass of epidiorite of the Cock’s Tor. 
The basic schists of the Lizards that have been so repeatedly discussed, 
are believed to have had a similar origin. 
Miscellaneous Notes.—The study of a series of nepheline rocks 
leads Gentil’ to the conclusion that the peg structure so characteristic 
of this mineral is an effect of alteration. The alteration product is 
often a hydrated pleochroic substance with a yellowish tinge. The 
‘ pegs’ are produced by the extension of this substance along directions 
of feeble cohension in the original mineral (solution planes ?) 
The rock by whose decomposition the apophyllite® of Callo in Algeria 
was formed, is a biotite-augite-andesite, whose groundmass is usually 
more altered than the phenocrysts. The inclusions found in the rock 
are of cordierite gneiss, fragments of andalusite and of sillimanite and 
large segregations of plagioclase a little more basic than the feldspar 
of the phenocrysts. 
On account of the similarity in crystalline structure between flint 
and Arkansas whetstone, Rutley’ is inclined to regard the latter rock 
as derived by the replacement of limestone or dolomite by silica. The 
rhombohedral cavities noted by Griswold are thought to have been 
ë Bull. No. 8, Geol. & Nat. Hist. Survey of Minn. 
* Quart. Jour. Geol. Soc., 1894, p. 338. 
1 Bull. Soc. Franc. d. Min., xvii, p. 108. 
"ib, p Ir 
? Quart. Jour. Geol. Soc., 1894, p. 377. 
