STRATIGRAPHY OF THE SKYKOMISH BASIN 561 



is the Index granodiorite in the northwest. The Cretaceous is not 

 represented but is known to occur as a marine series farther north- 

 ward. Following the Mesozoic batholithic intrusion came a posi- 

 tive orogenic movement, corresponding to the Laramie revolution, 

 which left the Cascade area, at the close of the Mesozoic, high 

 above sea-level. 



The Tertiary period opened with a period of continental deposi- 

 tion in which the Swauk sandstone was deposited unconformably 

 on the eroded edges of earlier metamorphic rocks. These arkoses 

 consist of mingled fragments of granitic rock and of schist derived 

 from the Paleozoic metamorphic series and from the Mesozoic 

 batholith. Both Smith and Willis consider these arkoses to be 

 lake deposits, but Weaver more correctly designates them as purely 

 continental. They show conglomeratic facies and cross-bedding 

 and vary so markedly in thickness that only the maximum figure 

 of 4,000 feet is of any significance. They are equivalent to the 

 lower Puget of western Washington and, on paleophytological 

 grounds, to the Fort Union of the Montana- Wyoming areas. No 

 definite information is recorded of the late Eocene and Oligocene, 

 the gap being succeeded by Miocene andesitic tuff beds. These 

 are volcanic tuff ejectamenta cut by dike- and sheet-like intrusions 

 of andesitic composition. We have insufficient evidence to suggest 

 where these tuffs had their immediate source, but their chemical 

 resemblance to the Miocene granodiorite suggests forcibly that they 

 were derived from the same magma. In later Miocene a recurrence 

 of deep-seated vulcanism took place, resulting in the injection of 

 the Snoqualmie granodiorite, which is the most important terrane 

 present in the area. This batholith, if its texture is to be accounted 

 for, must have been covered by at least 2,000 feet of rock. In this 

 cover late Eocene and Oligocene may be included. In latest 

 Miocene and in early Pliocene the region was planed to a low relief; 

 a disturbance of isostatic equilibrium followed, which resulted in 

 the arching up of the Cascade Mountains to a maximum height 

 of approximately 8,000 feet. Since this Pliocene uplift, canyons 

 some 5,000 feet deep have been cut in the granodiorite, a fact 

 bearing witness to the severity of erosion experienced by the 

 area. 



