ROCKS FROM NORTHERN AND CENTRAL CHINA. S73 
are full of inclusions of secondary minerals, but in those areas which show 
microcline structure, these inclusions are entirely absent and the mineral 
appears clear and fresh.* It is also noteworthy that these clear areas have 
a lower index of refraction than the decayed albite, thus indicating that 
the latter has undergone a chemical change as well as a structural one. 
Williams, in discussing the acid rocks of the Marquette districtt expresses 
the belief that the microcline is not recrystallized, but is merely a feldspar 
which has undergone mechanical deformation. Bayley, however, in treat- 
ing of the same rocks} inclines to the view that the microcline has entirely 
recrystallized, and in support of this idea cites its freedom from alteration 
products and its habitual occurrence as a component of the quartzose 
mosaic, which is evidently of secondary origin. I am inclined to think 
that Bayley is right in the majority of cases. But, as will be mentioned 
later, it seems highly probable that microcline does sometimes develop 
in orthoclase as the result of mere compression. 
In certain orthoclase crystals here, there are wavy branching seams 
filled with a feldspathic material which has a different orientation from 
that of the original crystal (Plate LIV, Fig. B). The seams are roughly 
parallel to one another, and are so arranged with reference to the zones of 
shearing in the adjacent rock as to make it seem probable that they were 
fissures developed by a tensional strain operating at a considerable angle to 
the prevailing direction of the seams. The hypothesis is here advanced 
that they are actually tension cracks which have been subsequently or 
simultaneously cemented with a more sodic feldspar. The higher index 
of refraction of this material shows that it is not orthoclase, but probably 
albite. In many cases it is possible to trace the material in the seams out 
into coalescence with the recrystallized feldspar of the mosaic surrounding 
the large orthoclase, thus showing that the filling of the fissures is identical 
with this mosaic. 
This granite shows evidence of mechanical strain in a higher degree 
than do those previously described. Along their edges the quartzes and 
feldspars are frequently granulated, especially in certain roughly parallel 
zones. Much of the detritus thus produced has recrystallized, forming 
streaks of clear quartz-and-feldspar mosaic. In these zones microcline is 
one of the commonest constituents. The quartzes which have not been 
actually granulated are elongated and show undulatory extinction very 
strongly, indicating that they are heavily strained. Many adjacent crystals 
have been ruptured and the parts separated, while others have been bent 
* Van Hise describes the same conditions in decayed orthoclase. (U.S. G. 5. Ann. Rep. xv, p. 503.) 
+ U.S. G. S. Bull. 62, pages 210 and 216, 
ft U.S. G. S. Monograph xxvut, p. 173. 
