130 



SCIENCE. 



[N. S. Vol. XI. No. 265. 



likely, however, either that the Potomac 

 deposits were removed from this region 

 prior to the Eutaw deposition, or else that 

 the surface of these old crystalline rocks 

 was above water level during Potomac time, 

 and hence not covered with deposits. 



Underground temperatures were not 

 taken at intervals at different depths while 

 the work was in progress, owing to the lack 

 of suitable thermometers ; but there are now 

 three wells only three or four feet apart, 

 one 1100, one 500 and one 100 feet deep. 

 The temperatures at the bottom of each of 

 these, as determined by the use of a Darton 

 deep well thermometer, were found to be 

 79°, 72.50°, and68.50°F. respectively, giv- 

 ing a descending increase in temperature of 

 about 1°F. for each 100 feet, between 100 

 and 500 below the surface ; and 1°F. for 

 each 98 feet, between 500 feet and 1100 feet 

 below the surface. 



J. A. Holmes. 



Chapel Hill, N. C. 



GRANITES OF THE SIERRA COSTA MOUN- 

 TAINS IN CALIFORNIA. 



The Sierra Costa mountains occupy 

 mainly the northeastern and northcentral 

 portions of Trinity county, in northwestern 

 California. They are the loftiest and most 

 scenic portion of the Klamath mountain 

 system, an off-shoot of the Sierra Nevadas. 

 They consist, in general, of highly met- 

 amorphic elastics and ancient igneous rocks, 

 including a basement crystalline formation, 

 a massive serpentine, and a series of mica- 

 ceous, chloritic, graphitic and hornblendic 

 schists. All these are pre-Carboniferous in 

 age ; they have been subjected to intense or- 

 ographic disturbance, folded and faulted on 

 a grand scale, and into the fissures have been 

 injected various granitic and dioritic dike 

 rocks. Of these, granite, in hugh batholites, 

 is by far the most important and bulky. 



Three principal types of granite are repre- 

 sented, and they present some interesting 

 contrasts : hence this paper. 



On the western side of the head-water 

 portion of the south fork of the Salmon 

 river in Siskiyou county, there is a huge 

 white mountain of nearby bare granite — Mt. 

 Courtney of the Cariboo range. It is a 

 massive batholite of true granite, consisting 

 of large individuals of quartz, white feld- 

 spar and dark brown biotite, but little or no 

 hornblende. It is very coarse-grained, the 

 three rock species being crystallized on a 

 scale of one-fourth inch. The color is a 

 very light gray, as a soda-feldspar is a 

 predominant constituent. 



The Courtney granite abounds in vein- 

 like dikes of aplite, a much finer grained 

 white granite, in which the biotite is in 

 small foils and sparingly developed. The 

 contrast between the massif of very coarse- 

 grained granite and the included dikes of 

 fine-grained aplite is strong. Evidently 

 they both represent the same magma, 

 but it seems that after the coarse 

 granite mass had solidified in its upper por- 

 tion, great fissures were formed in it and 

 the aplite arose in them, solidifying to form 

 the curious dikes of white granite. The 

 former is coarse-grained, because, being in 

 one great mass, it cooled slowly, and the 

 latter is fine-grained, because, being in 

 thin dikes widely scattered through an al- 

 ready solid rock, it cooled rapidly. 



Near the contact between the Courtney 

 granite and the hornblende schists on the 

 east, both granite and schist a»e cut by 

 dikes of a white muscovite granite, a kind 

 of fine-grained pegmatite. This contains 

 neither biotite nor hornblende, and is more 

 resistant to weathering influences than the 

 other granites of this area. These pegma- 

 tite dikes are cut by a transverse system 

 of dikes of dark green diorite-porphyrite, 

 which also occurs in the coarse-grained 

 biotite granite of Mt. Courtney, as well as 

 dikes of very fine-grained light greenish 

 gray diabase. 



On the east side of the head of south fork 



