300 The American Geologist. May, ism 
arate the slate belts into wedge-shaped portions, appear to 
have been intruded sheets or dikes. Such is the Bear moun- 
tain area that begins at the south end of the Jackson sheet 
and extends thence across Calaveras and Amador counties into 
Eldorado county, a distance of about fifty miles. 
This dike or intruded sheet (?) separates lor this entire 
distance, so far as known, the Calaveras and the Mariposa 
formations, and might be considered as a dike at their con- 
tact. At various points, particularly about Golden Gate hill. 
the diabase of this area extends in long dike-like tongues into 
the slates of both the Calaveras and the Mariposa formations. 
An inspection of the geological map of the area (Jackson atlas 
sheet) seems to leave no doubt that these intrusions cut 
across the sedimentary strata in places. 
No careful study has ye1 been made of these diabase areas. The 
dike-like tongues noted above may prove in all cases to be massive dia- 
base or augite-porphyrite, and the fragmental portions of these areas 
may prove to be interbedded. 
There are also large areas in which the material is unquestionable 
massive, as the area southwest of Latrobe.on the Placerville sheet, por- 
tions of which form an augite-porphj'rite. [n other portions of this area 
there are scarcely any bisilicates in the rock. No. 510 Eldorado is a 
sample of such a rock. This contains 68j{ of silica. 
In the northeastern pari of the Downieville sheet, in Plumas county, 
is a very extensive area of diabase. It forms a large pari of the western 
slope of the Grizzly mountains. The same area, apparently uninter- 
rupted, though in places covered by Pleistocene deposits, extends south 
to Mount Elwell, Gold lake ami beyond, having in places a width of six 
miles. A good deal of this material is breccia, anil most of it is more 
or less roughly schistose, port ions of it being converted into a t horoughh 
schistose amphibolite by pressure. A very pretty example of this 
d\ namometamorphism maj he seen just east of Long lake. Here a nar- 
row strip of the diabase has. b\ pressure and movement, been converted 
into an amphibolite-chlorite schist (Xo. lilt Plumas county); and that 
I his si rip of schist is an integral part of the diabase and not included 
in tie- diabase. i> \er\ evident, the rocks having been cleaned oil' mosl 
beautifulh bj the glacier thai formerly covered the area. Even better 
evidence of the dynamic origin of some of the amphibolite-schisl may 
be seen in the vicinity of Wade's lake, in Plumas county. On the slope 
to the south of the lake and at other points about the lake there are 
numerous diabase dikes in the white quartz-porphyry-schist. Some of 
these dikes are fifteen feet 1 h rolled) : ot hers are smaller. In places the 
dikes have assumed a schistose structure coincidenl in direction with 
the schistosity of the enclosing quartz-porphyry-schist, but bearing no 
relal ion to t he course of the dikes. 
