38 The Aiii(;)'ic(ni (Heoiof/isf. July, 1895 
(lueiitly (lescTibod b}' Prof. .1. K. Kemp :iiul the writer.* .1. 
Francis Williamsf has also contributed to the annual reports 
of the Geological Survey of Arkansas very elaborate petro- 
graphical investigations of similar intrusives found in the 
vicinity of Fourche mountain, Arkansas. In this locality 
were found all the members of the nionchiquite group. In 
defining the petrographical features of the monchiquites Dr. 
Williams states that "Rosenbusch considers these are made 
up of a porphyritic combination of olivine, augite and a glassy 
base with which ma}^ be associated either hornblende or mica 
or both of these minerals together. The glassy base often in- 
cludes minute crystals of plagioclase and occasionall}'^ of 
nepheline." 
FOUKCHITE DiKKS. 
In dikes Nos. 18 and 40, olivine, forming one of the essen- 
tial constituents of monchiquite, entirely recedes, thus pro- 
ducing an oli vine-free monchiquite. To such a mineral mix- 
ture J. Francis Williams has given the name fourchife.l 
The dike under consideration is composed chiefly of basaltic 
hornblende in two generations with a very subordinate 
amount of augite, a moderate amount of magnetite and a 
glassy base. The older hornblende phenocrysts are univer- 
sally large, well terminated and beautifully pleochroic. The 
smaller circular forms show the same feature as noted in the 
camptonites proper. Of the two generations the latter is 
more abundant. Augite is very subordinate, consisting of a 
few poorly developed individuals and occasionally pink or yel- 
low grains in a glassy base. Imbedded in the glass are 
many minute highly refracting needles. These may be apa- 
tite. A dark grayish opaque mass seems to be associated 
with the allotriomorphic augite. It may have resulted from 
the decomposition of augite, and may thus account for the 
unusually subordinate position of this mineral species. Much 
magnetite is also present. 
In the region of Fourche mountain, Arkansas, Dr. Wil- 
liams found numerous dikes of ver}" similar mineralogical 
composition. In these occurrences, however, augite seems to 
be the most prominent constituent, while hornblende is more 
*()p. cit. 
f.I. Fkancis Williams: Ijiiieous Rocks of Ark. Annual Report. Geo]. 
Survey ol' Arkansas, 18!)0, vol. ii. 
tSee Aiui. Rep. Geol. Survey of Ark., vol. ii, 1890, p. 110. 
