OUARRY MATERIALS OF NEW YORK 



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The cause of rift in the igneous rocks has been variously ex- 

 plained. Some writers have attributed it to a slight foliation pro- 

 duced by parallel arrangement of the mica minerals. In such 

 cases, it is comparable to the foliation cleavage of the metamorphic 



Fig. 5 Microscopic fractures in anorthosite, parallel to the rift or direction 

 of easiest cleavage. The section is nearly pure feldspar, the small grains 

 being garnet. Enlarged 25 times 



rocks. Another cause may be found in the regular arrangement 

 of the feldspars so as to bring their cleavages into alignment, as 

 has been described for a Norwegian syenite. Perhaps the more 

 common type of rift, specially in the granites, is that produced 

 by the presence of microscopic fracture lines. Tarr, Whittle and 

 others have noted many examples in which the cleavage arises 

 from very minute hairlike fractures, individually somewhat irregu- 

 lar and discontinuous, but in general holding their direction un- 

 changed throughout the rock mass. Such fractures are found in 

 both quartz and feldspar. Dale ^ more recently has shown that 

 rift may be related t'o minute cavities in the cjuartz, the cavities 

 being arranged in parallel sheets which, in some instances, are 

 accompanied by parallel fractures. 



Among the metamorphic rocks, a foliated or gneissoid structure 

 is usually accompanied by cleavage along the planes of foliation. 



1 U. S. Geol. Survey Bui. 354, p. 42-47, 1908. 



