224 SMITH AND MENDENIIALL — TERTIARY GRANITE 



of this paper as his assistant. The work was of the nature of a rapid 

 reconnaissance along the northern Cascades, yet during the brief stay in 

 the pass sufficient was seen to suggest the hypothesis that the granitic 

 rock here might be younger than the black slates, which contain fossil 

 plants. During the season of 1899 this area was visited by the authors 

 in connection with the survey of the Snoqualmie quadrangle. Detailed 

 mapping has afforded opportunity for the observation of facts which 

 conclusively prove the intrusive character of the granite. This determi- 

 nation of the age of the granitic intrusion is of value and interest in the 

 bearing it has on the history of the northern Cascades. 



General Geology 



s ed i men tary ro cks 



The sedimentary series exposed in Snoqualmie pass includes slates, 

 sandstone, and conglomerate. The slates are the most important and 

 cover large areas. The3 r are black or green, in the former case being 

 quite carbonaceous. Occasionally the slate has an irregular schistosity 

 and is knotted and crumpled, or it may become ver} 7 silicious and felsitic, 

 so as to closely resemble a hornfels. 



The sandstone occurs at a few localities bedded with the green slate 

 and is in part a white sandstone, only fairly well" cemented, and in part 

 a very quartzitic sandstone. A thin bed or lens of gray limestone aTso 

 occurs at one point associated with the slates. 



Beds of conglomerate in the black slate are quite noticeable along the 

 county road in the pass itself. It is of striking appearance, quite resem- 

 bling a volcanic breccia. The angular fragments, however, are all of sedi- 

 mentary rocks, with one possible exception. Black chert is abundantly 

 represented among these pebbles. 



The occurrence of fossil plants in the Snoqualmie Pass section is of 

 great importance to the geologist, although the matrix of these fossils is 

 not at all encouraging to the collector. The black slate here has little 

 cleavage developed and often breaks with a conchoidal fracture. The 

 material collected in 1895 by Mr Willis, however, afforded sufficient basis 

 for the determination of the series as Tertiary and probably Miocene. 

 Professor F. H. Knowlton's report on the collection was as follows: 



11 1 recognize four species of fossil plants, as follows : 



Platanus disseda Lesq. 



Acer sequidentatum Lesq. 



Finis n. sp.? cf. F. artocarpoides Lesq. 



Cinnamomum n. sp. 



