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THOMAS L. WATSON 



that biotite is greatly in excess, while quartz may be somewhat dimin- 

 ished in quantity. It frequently requires the closest examination, 

 megascopically, to detect more of the minerals than the dark ferro- 

 magnesian components, except in a few cases where an occasional 

 porphyritically developed feldspar has formed, so fine-grained and 

 very dark in color are the schlieren. 



Their distribution in the granites is subject to much variation. 

 From some quarries they are entirely absent, in others they are 

 developed only occasionally, and in others still they are so abundant 

 as to exclude the rock from use in certain higher grades of work. 

 Variation in size is from a fraction of an inch to more than a dozen 

 inches across. Irregularity of outhne is usually characteristic of 

 them, variation being from roughly oval-shaped and round to greatly 

 elongated areas, one of whose diameters is several times that of the 

 second. . 



MICROSCOPIC STRUCTURES. 



Peripheral shattering or granulation and recrystallization has 

 already been mentioned. It is only necessary here to call attention 

 briefly to two fairly constant microstructures in the granites, observed 

 in similar rocks elsewhere, namely, granophyric and micropoikihtic. 



Granophyric structure. — Granophyric intergrowths of feldspar and 

 quartz are either sparingly or abundantly developed in most of the 

 thin sections examined of the Carolina granites. They display the 

 usual form and development observed in granites in general. So 

 far as it was possible to determine, the intergrown feldspar may be 

 either of the potash or plagioclase species. The areas may be inclosed 

 by the larger feldspar individuals, or they may be formed at or near 

 the contact between the larger feldspar and quartz grains. Their 

 character clearly indicates simultaneous crystallization of the two 

 minerals — an overlapping in the period of formation of the feldspar 

 and quartz. This conclusion finds added confirmation in the micro- 

 poikihtic structure described next below. 



Micropoikilitic structured- — In thin sections of the granites from 

 both North Carolina and Georgia, certain of the large feldspar 



I G. H. Williams, "On the Use of the Terms Poikilitic and Micropoikihtic in 

 Petrography," Journal of Geology, Vol. I (1893), pp. 179 ff. This paper con- 

 tains numerous references to pubhshed accounts of this structure. 



