528 DEPARTMENT OF T\HE INTERIOR 



2 GEORGE V., A. 1912 



affected in its composition by the introduction of material from the granite. 

 The metamorphie effects are analogous to those observed about the xenoliths in 

 the Moyie sills of the Purcell range. 



In other xenoliths which show in their rounded outlines the corrosive effects 

 of the acid magma, a large number of peculiar round bodies have been developed. 

 These are of the size and shape of a small pea and, because of their special 

 hardness, they project above the general weathered surface of the xenolith. 



Under the microscope each of these small bodies is seen to be composed of 

 pure quartz, generally as a single crystal, but sometimes in the form of a coarse- 

 grained aggregate. The quartz nodules are perfectly clear and bear no inclu- 

 sions of the dioritic material. Between the diorite matrix and the quartz there 

 is usually a narrow aureole of idiomorphic orthoclase and plagioclase crystals. 

 These project into the quartz much as similar crystals project into vugs and 

 miaroles of other rocks. 



The origin of these silicious nodules is not clear. They can hardly be 

 regarded as filled amygdules of the ordinary type, but seem rather to represent 

 phenocry3tic growths in the xenolith after the latter had been softened by the 

 granite magma and been impregnated with silicious material from that source. 



Skagit Volcanic Formation. 



From the first summit west of the Skagit river to the summit of Custer 

 ridge (the main divide of the Skagit range), the Boundary line crosses a very 

 thick group of volcanic rocks which may be called the Skagit volcanic formation. 

 These rocks extend over at least twenty square miles in the belt and continue 

 unknown distances in the mountains to north and to south. 



The volcanic rocks characteristically weather into jagged peaks, knife-edge 

 ridges, and forbidding precipices, forming the highest and most rugged moun- 

 tains in this part of the Skagit range. (Plate 44, B.) Glacier Peak and its 

 neighbours are, indeed, among the most inaccessible summits in the whole 

 Boundary belt west of the Flathead river. Small but numerous glaciers and 

 a succession of impassable breaks in the ridges render the study of the volcanic 

 formation difficult even where outcrops are plentiful. Below tree-line it has so 

 far proved quite impossible to find a sufficient number of actual contacts or to 

 work put the succession of the many members of the group. The results of the 

 exploration are, therefore, far from being satisfactory. It is known that the 

 formation is exceedingly thick — apparently at least 5,000 feet thick at the 

 Boundary line — but the writer has been baffled in the attempt to construct a 

 detailed and final columnar section. The great thickness of the volcanic 

 accumulation and the abundance and coarseness of the agglomerates suggest 

 that the major eruptions actually took place in the area of the Boundary belt. 

 It is quite possible that a vast cone of Mount Baker or Mount Bainier pro- 

 portions was situated over the present site of Glacier Peak. 



The lower and greater part of the formation, probably 4,000 feet or more in 

 thickness, is composed of massive breccias and ash-beds, with one layer of coarse 

 conglomerate and with many interbedded flows of compact and vesicular lava. 



