STRATIGRAPHY OF THE SKYKOMISH BASIN 567 



dikes have baked the sandstones. The Keechelus is therefore 

 post-Swauk in age. Contrariwise, the Keechelus series has been 

 indurated and otherwise metamorphosed by the Snoqualmie 

 batholith to the southward; in the Snoqualmie area the Keechelus 

 series is underlaid by a sedimentary series of sandstones and water- 

 laid pyroclastics (Ellensburg) which contains flora called Upper 

 Miocene by Knowlton. 1 The Keechelus series in the Skykomish 

 Basin, being really continuous and lithologically identical with 

 the series of the southern area, is therefore considered Miocene 

 in age. As has been pointed out by Smith and Calkins, a striking 

 chemical similarity exists between the Keechelus andesite and the 

 subjacent body of granodiorite. Both fall into the same chemical 

 classification — tonalose. It is immediately inferred that the two 

 rocks are consanguineous, or, in other words, that the andesitic 

 pyroclastics were blown out from a magma that later solidified as 

 the Snoqualmie granodiorite. We must assume 2,000 feet or more 

 of cover, and it is suggested that this Keechelus series may very 

 well have provided at least a part of the cover which has later 

 been removed by processes of erosion. No estimate can be 

 made of the thickness of the Keechelus series. It is undoubtedly 

 widely variable and probably had a thickness of several thousand 

 feet. 



Snoqualmie granodiorite. — Into the Keechelus series was in- 

 truded one of the younger of the known great batholithic intrusions. 

 It has a known length of major axis of about thirty miles and is 

 approximately two-thirds as broad. Throughout, this terrane is a 

 massive, fresh, granitoid igneous rock which has been discovered 

 by erosion to a vertical depth of 5,000 feet or more. It has, as was 

 seen above, metamorphosed rocks of late Miocene age, and has 

 sent apophyses into them, and therefore must itself be Miocene or 

 later in age. To account for its holocrystalline nature and for the 

 fact that it has been peneplaned, uplifted, and maturely dissected, 

 it seems necessary to put the date of its intrusion as near that of 

 the Keechelus as possible. The age is therefore given as late 



1 G. O. Smith and W. C. Mendenhall, "Tertiary Granite in Northern Cascades," 

 Bull. Geol. Soc. Am., XI (1900), 224; G. O. Smith and F. C. Calkins, Folio 13Q, U. S. 

 G.S., 1906, p. 8. 



