INTRUSIVE NATURE OF -THE GRANITES 371 



the associated pegmatites are the bane of the quarryman and necessitate 

 much dead work and much waste. 



The basic inclusions. — A second and very strong form of evidence of the 

 irruptive nature of the granite is furnished by the inclusions of dark 

 basic rock, which are met in ahnost all the quarries. The inclusions 

 are of two kinds. The first, which consists chiefly of biotite with subor- 

 dinate orthoclase, plagioclase, quartz, apatite, and magnetite, shows no 

 foliation and is apparently a basic segregation from the magma. It has 

 merely the minerals of the granite with the biotite in great excess. The 

 other is a distinctly foliated, hornblende schist or amphibolite and pre- 

 sents long slabs, which are precisely like the hornblendic schists, occur- 

 ing in the countr}^ rock. A particularly fine example was exposed in 

 1895 in the quarry north of the railway station at Leets island. It was 

 a dark slab, about 8 feet long and a foot thick. Marked foliation ran its 

 entire length. It was black and in strong contrast with the red granite 

 that held it. A photomicrograph of a thin-section is shown in plate 39, 

 figure 2. Twenty feet away was another included mass, a block about 

 4 feet in diameter. Around the edges of the inclusions there were several 

 inches of coarse pegmatite before the normal granite was met. This 

 pegmatite border has been noted a number of times and is one of the 

 features of the foreign inclusions. Mineralizers have apparently gath- 

 ered in special amount around the included mass and have caused the 

 large feldspars and quartz to develop. Under the microscope the slab- 

 like inclusion from Leets island was found to be chiefly hornblende, 

 with which was associated considerable biotite, plagioclase, scapolite, 

 titanite, magnetite, and a little quartz. This assemblage is that of a 

 typical hornblende schist. A basic inclusion in the granite of the Brook- 

 lyn quarry, at Stony creek, was found to have undergone alteration and 

 to have yielded red rhombohedra of chabazite. 



These inclusions correspond closely to the basic country rock, and 

 there seems no reasonable explanation for them other than that they 

 have been torn off from these originals during the intrusion of the 

 granite. 



Some special emphasis is placed on the intruded character of the 

 granites because they have been regarded as extreme phases in the met- 

 amorphism which in its incomplete development has produced the 

 gneisses. Although nowhere stated in so many words, the late Pro- 

 fessor Dana seems to impl}' this view in his interesting geological guide 

 book to the environs of New Haven.* The writer, hesitates to differ 

 with so respected and generally beloved an authority as Professor Dana, 

 but in this particular he only reflected the prevailing views of an earlier 



*J. D. Daaa: The four rocks. New Havea, 1891, p. 118. Professor Dana did not overlook the 

 effects of dynamic metamorphism, but does not consider the granites irruptive, 



