490 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



3 mm. in diameter. From the rather basic form described the rock 

 varies to types which are almost white and consist of scattered 

 prisms of hornblende in a feldspathic base. In places near the 

 lower contact there is a very marked lamination, the rock consist- 

 ing of layers of feldspathic material alternating with layers com- 

 posed almost entirely of prismatic hornblende. This may be due 

 to flow structure, segregated bunches of mafic minerals having been 

 drawn out into lines. The fact that this lamination is represented 

 only at the bottom contact where the dike is almost horizontal makes 

 it interesting to consider the possibility that these alternating laminae 

 may be due to crops of crystals of ferromagnesian and of felds- 

 pathic minerals separating alternately from the mass of the dike 

 and sinking through the still fluid magma to the bottom. The 

 most acid phase of the rock is somewhat pegmatitic in character and 

 clearly represents an acid residuum which concentrated in patches as 

 the last material to crystallize. 



Under the microscope the main mass of the dike is seen to consist 

 of a beautifully trachytic aggregate of laths of plagioclase and 

 prisms of greenish-brown hornblende. Small shreds of pleochroic 

 brown biotite are now almost entirely altered to chlorite. The 

 feldspar, which is the most abundant mineral, is a plagioclase which 

 occurs in beautiful laths characterized by the rarity of twinning on 

 the albite law, the majority of the crystals being simply twinned 

 once. In the main mass of the rock the feldspars are all beautifully 

 zoned and range in composition from basic bytownite, Ab^g Augi, at 

 the center to acid andesine, Ab-^ Aujo, at the peripheries. Where 

 the rock contains porphyritic phenocrysts of feldspar, these are 

 usually even more basic than in the main mass of the rock, ranging 

 down to pure anorthite in the center. These beautifully zoned 

 phenocrysts frequently show carlsbad, albite, and pericline twinning 

 in the same individual. Occasionally there are inclusions of quartz 

 which reach 3 mm. in diameter, each inclusion being a single crystal 

 individual. These are commonly surrounded by rims of hornblende 

 and frequently have a small patch of chlorite in the center. The 

 quartz contains lines of fluid inclusions. The hornblende is in 

 prisms which are somewhat ragged and frayed at the ends but which, 

 in cross sections, are sharply euhedral. The hornblende is fre- 

 quently twinned parallel to (100). It is a basaltic variety, pleo- 

 chroic in pale to dark greenish brown. In the coarsest-grained acid 

 phase of the rock the minerals present are the same as in the main 

 mass of the dike — plagioclase, hornblende, and some biotite. The 

 hornblende is pale to dark brown, lacking the greenish tinge of the 

 finer-grained rock. The biotite, which is much altered to chlorite, is 

 also pleochroic in brown tones. It is present only in rare shreds. 

 The feldspar, which is greatly kaolinized, is not so basic as in the 



