1896.] Petrography. 1039 
peridotites, occur in small Jenticular masses or in narrow strips, which 
are always enveloped in a sheet of schistose talc or chlorite, and thus 
are never in direct contact with the gneisses through which they are 
believed to cut. They are classed as peridotites, pyroxenites and am- 
phibolites, the former being the most common, The peridotites pre- 
sent several types in each occurrence, all merging into one another and 
forming a single geological unit. The principal types of the perido- 
tites are dunite, harzburgite, amphibole-picrite and forellenstein. All 
are massive, as a rule, though exceptions arenoted. The dunite is com- 
posed of olivine grains, octahedrons and rounded grains of picotite and 
chromite, plates of enstatite, prisms of light green hornblende and 
various alteration products of these, the most common being serpen- 
tine tremolite and chlorite. The Harzburgite and the other perido- 
tites present no unusual features. They appear to be transition phases 
between the dunite and the various pyroxenites among which are rec- 
ognized two types, an enstatite rock and websterite. The enstatite 
rock is made up almost exclusively of enstatite or bronzite and its al- 
teration product tale. An analysis of the enstatite gave : 
SiO, Al,0, FeO CaO MgO MnO H,O Total 
51.64 12 9.28 .45 31.93 .56 5.45 99.43 
The amphibolites are composed chiefly of amphibole. The most im- 
portant type is composed of grass-green hornblende, anorthite and 
more or less corundum. The rock is fine grained and it is usually 
gneissic, although occasionally massive. Transitions through forellen- 
stein into dunite were observed, although the distribution of the rock 
occurrence in a system of dykes cutting the latter rock. 
The hornblende has the following composition : 
SiO, Al,O, Cr,O, FeO NiO MgO CaO NaO K,O H,0 Total 
45.14 17.59 .79 3.45 .21 16.69 12.51 2.25 86 1.34 — 100.33 
Genth called the mineral smaragdite. Dana regards it as edenite, In 
addition to the rocks mentioned above, there are also present in the 
region massive serpenite, which was unquestionably derived from dun- 
ite, tale-schists, and soapstones derived from enstatite rocks and chlorite 
schists. - 
In a second paper* the same author gives his reasons for considering 
these rocks as eruptive in origin. 
Shales and Slates from Wales.—-Hutchins’ continues his 
studies of clays, shales and slates by an investigation of the nature of 
* Elisha Mitchell Sci, Soc. Jour., Pt. II, 1895, p. 24. 
5 Geol. Mag., Vol. ITI, 1896, p. 
