REPORT OF THE CHIEF ASTRONOMER 623 



SESSIONAL PAPER No. 25a 



of the ridges in soft rocks, while the hard rocks, presumably in this case 

 the granites, retained more nearly their original elevation. It may be said 

 in this connection that field observations do indicate that the granitic 

 rocks of the Cascades are in general more resistant than the associated 

 schists, serpentines, slates, etc. Again, the general level of the Cascade 

 Plateau as it exists at present corresponds, approximately, with the timber 

 line as determined by existing climatic conditions. As weathering is more 

 active above timber line than below it, we have, perhaps, additional reason 

 to assume that the Cascade peneplain, raised, as we have assumed, to a 

 position about 10,000 feet above the sea, has in general been lowered to the 

 horizon of the timber line, leaving the more resistant granitic rocks in 

 relief. There are thus several arguments which it may be claimed tend to 

 show that the surface of the Cascade Plateau was formerly higher than 

 it is now and that it has been lowered by erosion, but to me the evidenw 

 seems far from conclusive. 



' Another tentative explanation of the greater prominence of the 

 granitic mountains over their neighbours of schist, etc., calls for local 

 upheavals since the Cascade peneplain was raised into a plateau and sub- 

 sequent to the initiation of the present master drainage lines. That is, 

 if we assume that the granitic cores of the mountains have been pushed 

 upward since the plateau was raised to its present general elevation of 

 about 7,500 feet, all of the observed facts bearing on the question tinder 

 discussion fall in line and find a mutual explanation.' 



Russell was inclined to consider the latter hypothesis as the more probable 

 one. After _noting the evidences which are acknowledged not to be convincing, 

 he proceeds : — 



'Briefly stated, my conception of the origin of the larger topographic 

 features of the northern Cascades is that the region, having a complex 

 structure, was reduced by erosion to a condition of low relief and at a 

 later time than the folding of the Tertiary sediment and the outspreading 

 of the Columbia lava was broadly upraised about 7,500 feet in the axial 

 region. The courses of the larger streams were then established and the 

 plateau was deeply dissected. During this later cycle there have been 

 movements in the rocks which, as a part of their results, have raised 

 certain of the granitic areas above the general level of the plateau. . . . 

 ' The date of the period of planation is shown approximately by the 

 fact that folded beds of Eocene age were truncated. The broad peneplain 

 must, therefore, have reached its greatest degree of perfection in late 

 Tertiary time, probably extending into the Pleistocene. 



' After the time of long-continued erosion referred to above, when the 

 Cascade region in northern Washington was reduced to a peneplain, there 

 came a time of elevation, when the peneplain, or a very large portion of 

 it, was bodily raised some 7,500 feet at least, and thus became a plateau. 

 In a broad view of the region this Cascade Plateau may be considered as 

 of the nature of a broad, flat-topped anticline, or as Dana would probably 

 have called it, a geanticline.' 



