152 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



Though much fewer in number than the quartz pebbles, angular fragments 

 of feldspar are seldom wanting from the conglomerates and coarser grit beds. 

 The microscope shows them to be orthoclase (or microcline), microperthite, and 

 basic andesine, named in their apparent order of importance. Where the feld- 

 spars are specially abundant, the grit has the look of a metarkose. The feld- 

 spar is usually more or less kaolinized or sericitized. Slate fragments are 

 always relatively rare and probably never make up more than five per cent of 

 the whole number in any one bed. 



The cement of the conglomerate and grit is a variable mass of sericite, 

 and fine-granular quartz, with which minute foils of biotite may be associated; 

 magnetite forms a never failing though not abundant accessory; chlorite, 

 zoisite, tourmaline, and sillimanite are other constituents, always in small 

 amounts. 



From the conglomeratic phases there are all transitions to the only less 

 important interbedded sandstones and metamorphosed sandy argillites. The 

 sandstone may, in fact, be regarded as but finer-grained equivalents of the con- 

 glomerate, while the altered argillites are more highly micaceous, compact 

 analogues to the cement of the conglomerate. Feldspar grains appear to be 

 very rare in these finer-grained phases. The well water-worn grains often 

 afford beautiful examples of secondary enlargement whereby these rocks 

 have become very strong and resistant both to the hammer and the weather. 



Where the rock is fractured, the surface of fracture, as in a true quartzite, 

 passes indifferently through quartz grain and cement. The minute mica plates 

 and shreds strongly tend to be developed in planes of schistosity. These planes 

 pass clear through the clastic grains of quartz in such a way that a large grain 

 is flanked by two swarms of similarly orientated mica-foils, as shown in the 

 accompanying Figure 11. Thus, the micas as a rule do not wrap around the 

 clastic grains but are grouped in straight lines or zones which are cut off 

 sharply by the grains. It is clear that in this case the schistosity produced 

 by the common orientation of the micas is not due either to shearing of the 

 rock or to the rotation of pre-existing sericite and biotite but is due to the 

 crystallization of these minerals with their cleavages lying perpendicular to 

 the direction of a compressive force. 



The schistosity is almost always parallel to the bedding. Part of the 

 metamorphism may have taken place after the old sediments were turned up 

 on edge. However, the fact that the flat-lying sandstones and argillites of the 

 Lewis, Galton, and Purcell series show similar fissility and recrystallization, 

 seems to indicate that most of the recrystallization of the Wolf and overlying 

 formations was completed before the upturning. In the present case tangential 

 force simply completed a process which had been nearly finished under con- 

 ditions of static metamorphism. 



The microscope shows that the feldspar of the coarser sandstones is char- 

 acteristically microperthite or microcline. Orthoclase and plagioclase are very 

 rare and generally seem to fail altogether. The microperthite, like the micas 

 (sericite and biotite) and much of the qu'artz, shows evidence of having de- 



