STRATIGRAPHY OF THE SKYKOMISH BASIN 569 



slight soil covering at present — an evidence of recent volcanic 

 activity in the near-by volcanic cones. 



RESUME 



Eocene time witnessed the accumulation of 4,000 feet of arkose 

 sandstone. This was orogenically disturbed so that it now dips 

 at considerable angles. In the Miocene a series of volcanic tuffs 

 and andesitic intrusives were derived from a magma which ap- 

 proached the surface and cooled as a batholith in late Miocene. 

 Pliocene saw the formation, uplift, and mature dissection of a 

 peneplane. The Pleistocene was a period of glaciation lasting 

 nearly to the present, in which the Skykomish area was maturely 

 dissected by ice erosion. 



IV. STRUCTURE 



A glance at the map shows the tendency of all formations to 

 trend north-south. Only the Swauk arkose series deviates from 

 this tendency, and even this formation tends to assume a normal 

 relation in its northward outcrop. The great igneous terrane 

 (Miocene batholith) has its major axial trend in a direction parallel 

 to the north-south axial trend of the Cascade Range. 



JOINTING 



There are two known systems of joints of considerable impor- 

 tance and one or more of less prominence. The first two strike 

 N. 45 E. and N. 70 E., respectively, and the lesser system strikes 

 N. 8o° W. It is suggested that these joints are the result of pres- 

 sure exerted by orogenic forces in raising the Cascade peneplane 

 to its present position. If this pressure were exerted continuously 

 in an east- west line from the Pacific side, we should anticipate a 

 set of joints striding N. 45 E. and a lesser set striking N. 45 W. 1 

 At least one important set of joints does strike N. 45 W. in the 

 Cleopatra Mine. But it is necessary to postulate a change in the 

 direction of application of the force to account for the system of 

 joints striking N. 70 E. and N. 8o° W. 2 It seems possible that 

 such a change may have taken place in the direction of application 



1 A. Daubree, Geologie experimentale (1879), PP- 3*6 f- 



2 G. F. Becker, "Finite Strain in Rocks," Bull. Geol. Soc. Am., IV (1893), 23. 



