REPORT OF TEE CHIEF ASTRONOMER 531 



SESSIONAL PAPER No. 25a 



meen series of sediments. If the latter are of Carboniferous age and the granite is 

 Jurassic, the Skagit volcanics rest upon a late Jurassic or post-Jurassie erosion- 

 feurface. The relation is somewhat similar to that between the volcanic breccia 

 at the base of the Pasayten formation and the underlying, probably Jurassie 

 Pvemmel batholith. 



There is something, therefore, to be said for the hypothesis that the Skagit 

 Tdcanics are of Lower Cretaceous age and contemporaneous with the Pasayten 

 volcanics. If, however, the Custer batholith was intruded in the late Jurassic 

 and sheared and metamorphosed during the orogenic revolution at the close of 

 the Laramie period, it would seem certain that the Skagit volcanics must belong 

 in the Tertiary. This follows from the fact that the volcanic rocks are com- 

 paratively little disturbed and are nowhere sheared in anything like the measure 

 shown in the Ouster gneissic batholith. It would seem impossible that the 

 basement could be so profoundly affected while the thick cover should escape 

 the deformation. That the Skagit volcanic formation is not younger than the 

 Miocene is probably indicated by the fact that it is cut by a stock of quartz- 

 bearing monzonite, which shows evidence of being essentially contemporaneous 

 with the Castle Peak stock (late Miocene). At present the dating of the vol- 

 canics cannot be made any closer with definiteness. In the correlation tables 

 the writer will postulate an Oligocene date for them, thus equating the Skagit 

 andesite with the proved Oligocene andesite in the Midway district. The 

 Skagit andesite may, on the other hand, be Eocene or, possibly, Cretaceous. 



Skagit Harzburgite. 



On the ridge 2,500 yards north-northwest of Monument 67, at the 6,600- 

 foot contour, the Custer gneiss is cut by a large pod-like intrusion of coarse 

 peridotite. This mass is 150 feet or more in width and can be followed along 

 its longer, north-south axis about 900 feet. It appears to taper off -toward each 

 end. It is probably an irregular dike injected into a schistosity-plane of the 

 gneiss. From wall to wall the peridotite is very coarse, showing olivines often 

 reaching 2 cm. in diameter and an abundant pyroxene of similar dimensions. 

 At the ledge the rock is seen to be somewhat altered, but it shows no sign of 

 crushing. 



The general colour of the rock is a deep, almost blackish, green. Feldspar 

 i3 entirely lacking. In thin section the composition and structure are seen to 

 be that of a typical, partly altered harzburgite. The only primary minerals 

 are olivine and enstatite, both colourless in thin section. About fifty 

 per cent of the rock is made up of secondary minerals, including serpentine, 

 tremolite, iddingsite, talc, chlorite, much sulphide (probably pyrite), and con- 

 siderable limonite. Minute inclusions of picotite or chromite could be discerned 

 in the olivine. The iddingsite noted has most of the features described by 

 Lawson for the type material at Carmelo bay, but the optical angle is very small, 

 2V being well under 5°. The specific gravity of the rock described is 3-083. 



The date of this intrusion is apparent only in relation to the period when 

 the Custer batholith was sheared; the shearing seems to have been completed 



25 a— vol. ii— 34£ 



