S6 ' 'File American Geologist. Febmary, isoo 
dioryte-porph\Tyte. This is a very fine-grained crystalline 
of dark gray color, easily decomposed into a dark brownish 
sandy mass from which it has received the miners' name of 
"sand porphyry." It is not largely represented either in 
large or as numerous dikes, but as it occurs in a significant 
relation to several of the richest gold-bearing lodes, its im- 
portance is out of all proportion to its bulk. Although so 
nearly related in composition to the dioryte-porphyryte 
characterised by hornblende needles, it forms quite a dis- 
tinct system of, I believe, considerably later age. Indeed, 
it seems to be the latest of the intrusive and may be as re- 
cent as the mineral veins with which it is generalh- con- 
nected. 
Diabase. — Great masses of fine-grained, light greenish 
gray diabase occur along the axis of the Cinnabar synclinal 
east of the main Trinity river, where they may be a portion of 
the Jurassic diabase folded into the serpentine and "picrolyte'* 
series. There are true dikes of a similar diabase in the Upper 
Cofifee creek region, occurring as narrow, vein-like sheets in 
the Courtney granite, and the other rocks of the district. They 
are newer than the entire granitic series, but their relation to 
the dioritic series is obscure and not yet determined. How- 
ever, they are certainly much later in age than the Jurassic 
diabase, thus demonstrating that, in northwestern California, 
there was more than one period of diabasic intrusions. 
Physiography. 
The remarkable correspondence in hight of all the principal 
peaks throughout the Sierra Costa region, is suggestive of the 
idea that they are remnants of an ancient peneplain. That such 
a base level of erosion is due at or a little above their summit- 
plane is evident from a study of the character of the drainage 
system. Except to a very minor degree, the streams are ab- 
solutely independent of the structure of the mountain forma- 
tions. For instance, Coffee creek rises on the w-estern slope of 
the second or western anticlinal, flows northwardly along that 
slope, then turns at a sharp angle and makes its way obliquely 
across the whole width of the structural "elevation," with its 
two sharp anticlines and deep Cinnabar syngline. Indeed, 
most of the large streams, including the East fork, the main 
