84 J. C. BKANNER GEOLOGY OF NORTHEAST COAST OF BRAZIL 



little west of Antonio Olyntho, and granites are also reported by him 

 between Sao Bento and Pesqueira farther west. 



Since the preceding pages were written sections of the jointed and 

 felted rocks found near Antonio Olyntho were submitted to Dr J. P. 

 Iddings, who kindly writes of them as follows : 



" The three sections sent me appear to come from the same rock. Under the 

 microscope it is seen to be a metamorphosed granitic rock, or possibly granitic 

 porphyry or a rhyolite. It consists of phenocrysts or the remnants of potash 

 feldspar in a micro-granular groundmass of quartz and unstriated alkali feldspar. 

 There is a sheared, streaked, flow structure, and it is plainly seen that there has 

 been crushing and shearing and recrystallization of the minute crushed particles. 



\'(4 w '^%. 





Figure 15. — Jointed and metamorphosed Rhyolite Tuff near Antonio Olyntho. 



The large feldspars are partly shattered and split apart and the spaces betweeu 

 filled with granular quartz. The feldspars are microcline by the straining of ortho- 

 clase, the twinning being most noticeable where the stress appears to have been 

 great. The minute grains of feldspar are unstriated, with low refraction, therefore 

 alkali, presumably orthoclase (?) (unstriated microcline). In places the large un- 

 twinned feldspars are streaked with another unstriated feldspar in a perthitic 

 manner. The grains of quartz and feldspar in groundmass are not evenly distrib- 

 uted, but are in streaked patches of many feldspar grains and many of quartz, 

 suggesting that the original rock was a fine grained granite, rather than a porphyry. 

 " There is very little of other minerals present. Small particles of colorless and 

 yellowish muscovite in streaks, small crushed apatites, little crushed magnetite, 

 little (?) hornblende, pale green. I should think the rock a crushed orthoclase 

 granite, having very little mica and little else." 



