METAMORPHISM 227 



in the texture of the rock is noticed. As a whole the granite becomes 

 finer grained, while the quartz becomes a more conspicuous constituent. 

 The variations are gradual, until at or near the contact the rock is a 

 granite porphyry, with blebs and phenocrysts of quartz, both abundant 

 and prominent. 



The rock of the dikes and apophyses from the main mass is a granite 

 porphyry, as has been stated above. Indeed, the porphyry so closely 

 approaches the rhyolitic type that in certain occurrences the dike rock 

 is with difficulty distinguished from the effusive rock which occurs at 

 other points associated with pyroclastics. 



METAMORPHISM OF THE SEDIMENTARY ROCKS 



Eocene and Miocene strata occurring in adjacent areas in the Cascade 

 region show few traces of metamorphism. The sandstones may be loose 

 and friable, or with the associated conglomerates and shales they may 

 have been consolidated to such a degree as to form bold escarpments 

 thousands of feet in height; but even where intruded by countless 

 basaltic dikes, these Tertian sediments can not be said to be metamor- 

 phosed. In the Snoqualmie Pass area, on the other hand, the sediment- 

 ary rocks, though also of Tertiary age, appear much older. Their strat- 

 ification is generally difficult to determine. The shale becomes a compact 

 and flinty slate, while the sandstone has often the glassy and dense 

 character of quartzite. 



More striking even than these lithological characters of the rocks is 

 their influence upon the topography. So bold are the peaks composed 

 of these Tertiary slates and sandstones that it is usually impossible to 

 distinguish them from the topographic forms cut out of the jointed 

 granite and even harder granite porphyry. Sharp needle-like details of 

 a crestline may indicate either the sedimentary or the igneous rock. 

 Neither is the appearance of the talus bordering the steep slopes any 

 criterion as to the nature of the rock forming the cliff above. 



This metamorphism of the sedimentary rocks is, in part, of a dynamic 

 character. The presence of well developed joint planes has much to do 

 with the resemblance of the sedimentary rocks to the granite, as expressed 

 in the topographic forms developed : yet the greater part of the meta- 

 morphism appears to be traceable to the intrusion of the granite, since 

 it is in this contact region that the characters described above are best 

 developed. 



Furthermore, bordering the area of the granitic rock there is what 

 may be termed a contact zone. As the contact is approached from the 

 area of sedimentary rocks the increase in degree of metamorphism is 



