584 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



In the Norm classification the rock enters the dosodic subrang, andose, of 

 the alkalicalcic rang, andase, in the dosalane order, germanare; but is near the 

 corresponding subrang of the docalcic rang, hessase. 



In the older classification the rock is a hornblende-biotite diorite. Miner- 

 alogically and chemically it is almost identical with a California diorite des- 

 cribed by Turner.* The specific gravities of six fresh specimens vary from 

 2-T36 to 2-863, averaging 2-806. 



The apophyses of the body are chemically similar but have the structure 

 of hornblende-biotite diorite porphyrite. 



Contact Metamorphism. — The Paleozoic sedimentaries cut by the diorite 

 have been decidedly metamorphosed. The effects were noticeable at all of the 

 observed contacts, but were specially studied on Pierce mountain which forms 

 part of the rugged divide running southward between Slesse and Middle creeks, 

 and again along the contacts on Middle creek. The belt of altered rock seems to 

 average at least 600 feet and may be 1,000 feet or more. (Plate 44, C). 



Mineralogically the metamorphism shows nothing very unusual. The 

 sandstones have been converted into tough, vitreous quartzites. Some of the 

 argillites have been changed into dark greenish-gray hornfelses or schists, 

 richly charged with metamorphic biotite and sericite. Other argillaceous beds 

 have been recrystallized, with the generation of abundant cordierite, that mineral 

 forming, as normally, large, interlocking individuals which are filled with 

 inclusions of quartz, biotite and magnetite. A few thin lenses of pale green, 

 felted tremolite and of more granular tremolite and epidote probably represent 

 completely altered beds of limestone; other limestone bands have been changed 

 to white marble. With the limestones much chalcedonic silica is often asso- 

 ciated. 



The contact-belt is often traversed by small quartz-veins, some of which 

 form fairly high grade, free-milling gold ore. The Pierce claim on Pierce 

 mountain is located on one of these veins, close to the main contact of the 

 diorite. Like all the others seen in the vicinity this vein is quite variable in 

 width, pinching out, irregularly from its maximum width of a few feet. At 

 the time of the writer's visit to the claim, in 1901, not enough development work 

 had been done to show the amount or average value of the gold-bearing quartz. 

 A similar, though narrower vein, nine to twenty-one inches wide, cuts the diorite 

 at a point about 700 feet above Middle creek and 3,000 feet or more below the 

 main claim on Pierce mountain; the two veins may be connected, and both 

 were being opened up by Mr. Pierce in 1901. From the writer's experience the 

 veins occurring along this contact must be very high grade if they are to pay 

 for their development; they are much too small and irregular to give hope of 

 profitable low-grade ore. 



Ohilliwaok Granodiorite vBatholith. 

 Some of the wildest and most rugged mountains in the Skagit range are 

 composed of a massive granodiorite which forms the largest intrusive area in 



* See Bulletin 228, U. S. Geol. Survey, 1904, p. 234. 



