WARREN. — ALKALI-GRANITES AND PORPHYRIES. 213 



crystal. It often extends out with the same orientation in the form 

 of rather sharply bounded projections into the adjoining microperthite 

 grains. When developed along the sides, it not infrequently sends out 

 little prongs and hooks into the adjoining crystal much in the manner 

 described by Frisson ^'^ for the albite in the perthite of the Red Hill, 

 New Hampshire nephelite-syenite. The microcline member is twinned 

 almost exclusively after the albite law only, and therefore, lacks the 

 characteristic "gitter" structure usually associated with that mineral. 

 The twinning lamellae are relatively shorter and broader than those 

 of the albite member and their boundaries lack sharpness in most 

 cases. The twinning in the microcline is not uniform, even in a single 

 individual, and considerable portions may show little or no twinning. 

 Its optical properties indicate that it is a nearly pure microcline and 

 not a soda-rich variety. The intergrowth as it stands is apparently 

 a mixture of nearly pure albite and potash feldspar. 



A small amount of albite occurs in the rock in the form of separately 

 crystallized grains occurring along the sides of the larger crystals, or 

 in the interstices between them. There is often a suggestion, however, 

 that many of these were originally continuous growths with the albite 

 of the adjoining microperthite crystals, and that they have subse- 

 quently been deorientated by a slight movement' in the mass. 



The relations of the two feldspars detailed above is the normal 

 relation seen without substantial \'ariation in a great number of thin 

 sections from the granites of the quarries. In the granite from various 

 parts of the area and particularly from certain quarries, consideral:)le 

 variations from this texture, howe^'er, may be seen. The micro- 

 perthite frequently contains random crystals of albite, often in con- 

 siderable abundance. These are usually very small and have a slightly 

 elongate, prismoid habit but do not possess sharp terminations. 

 Again the microperthite may contain much larger, curiously irregular 

 grains of albite, often including perthitically intergrown microcline. 

 A number of these albites, much interdented along their contacts, may 

 replace a good portion of an original feldspar crystal. Still again the 

 microperthite may be replaced in part or almost entirely by aggregates 

 of elongated, parallelly arranged, mutually interdented crystals of 

 albite. Some potash feldspar may usually be noted intergrown with 

 the albite. These crystals are often obviously arranged with their 

 longer axes parallel to the original direction of perthitic intergrowth, 

 but not always. They appear sometimes in the center of a micro- 



12 American Journal of Science, 23, p. 272. 



