NATURE OF INTRUSION 229 



the facts now in hand. On the summit of Denny mountain, 5,766 feet 

 above sealevel, the granite is a rock of medium grain, showing none of 

 the textural variations that are seen on the northeastern slope of this 

 peak at a distance of half a mile from the contact with the sedimentary 

 rocks. At another locality, at the southern base of Huckleberry moun- 

 tain, the granite preserves its uniform texture close to the contact, while 

 a dike extending into the overlying rock shows the usual change in 

 texture. Here the cover of volcanic rock is at present about 2,000 feet 

 in thickness, and it is not thought that its thickness ever exceeded 3,000 

 feet. The physical character of the rock under which the magma con- 

 solidated is doubtless a factor that renders the problem a far from simple 

 one when the attempt is made to ascertain the conditions attending this 

 late intrusion of granite. 



Granitic rocks similar to that here described are known to occur 

 along the crest and western slope of the Cascade mountains as far north 

 as the International boundary, and also as far south as Mount Rainier.* 

 At the latter locality an old granite ridge forms an elevated platform, 

 on which rests the volcanic cone. The Rainier volcano is relatively so 

 recent that these relations do not preclude the possibility of correlation 

 of the granite with that described above. 



On the eastern slope, in the quadrangle adjoining this on the east, the 

 Mount Stuart granodiorite is known to be of pre-Eocene age, and this 

 granitic rock is also of probable areal importance to the north and 

 northeast. 



The Mount Stuart granodiorite forms the core of the Wenache moun- 

 tains, an extremely rugged range transverse to the general trend of the 

 Cascade mountains. This rock is intrusive in serpentine and other pre- 

 Eocene formations, and here also the intrusion appears to be of a batho- 

 lithic nature. These two granitic rocks are quite similar in general 

 composition and appearance, and the Mount Stuart granodiorite exhibits 

 no gneissoid structures that would indicate its greater age. The two 

 intrusions, the one of late Tertiary and the other of pre-Tertiary age, 

 must, however, be separated in the geological study of the northern 

 Cascades. Together they are believed to constitute perhaps the most 

 important geologic feature in this range, and as such they must be 

 taken into consideration as the growth of the Cascade range is traced. 



t Resume 



The granitic rock of Snoqualmie pass is intrusive in sedimentary 

 rocks of Tertiary age. Its intrusive nature is made evident by the dikes 



*18th Ann. Rept. U. S. Geological Survey, part II, p. 422. 



