384 
The American Geologist, 
June, 1896 
somewhat different structure and composition, but the vari¬ 
ation is small. The porphyry is made up chiefly of interlock¬ 
ing grains and prisms of albite, often untwinned, and in some 
sections there are definite phenocrysts of polysynthetically 
twinned feldspar (probably likewise albite) developed. 
Rather abundant in some specimens is an olive-green mineral, 
in grains and prisms. The extinction is sensibly parallel to 
the prism, and the interference colors are bright. Good cross 
sections showing beyond doubt the nature of the cleavage 
were not found, but so far as could be made out the cleavage 
is similar to that of mgirite, and it is thought to be that 
mineral. These dikes, therefore, show some analogy to certain 
dikes in Norway, described by Brogger, and called by him 
solvsbergite. The variety found near Lougenthal* is com¬ 
posed chiefly of albite, but contains also microcline, hornblende 
(katofori’te and some arfvedsonite), mgirite, traces of biotite, 
and quartz, accessory apatite, and zirkon, and occasional 
minute grains of a perovskite-like mineral. 
Dikes of syenite-porphyry were also noted in some siliceous 
schists or thin bedded quartzite of presumably Paleozoic age 
to the north of the Merced river. One of these dikes about 9 
miles southeast of Coulterville in a much decomposed con¬ 
dition, can be traced for a mile or more with a strike to the 
west of north. The soft, decomposed dike material has 
apparently been auriferous as numerous cuts have been made 
in it, and claims located. As this particular dike was not 
investigated chemically or optically it is quite possible that 
it contains some free quartz, and is more nearly related to No. 
1639, than to the soda-syenite porphyry of Moccasin creek. 
In the canyon of the Merced, on the steep slope west of the 
river just down stream from the mouth of the north fork, are 
from fifteen to twenty white dikes from six inches to six feet 
in width. These dikes are nearly vertical following the dip of 
the schists (75° E.) but at other points cutting them at a 
slight angle. This group of dikes may also be called a mul¬ 
tiple dike. A section from a similar dike probably a contin¬ 
uation of one branch of the multiple dike, taken just opposite 
on the east side of the river (No. 1639), shows a few twinned 
*W. C. Brogger : Die Gesteine der Grorudit-Tinguait-Serie, Chris¬ 
tiana. 1894, p. 76. 
