DIKES AT HEAD OF MIDDLE CREEK. 313 



dorite, probably with the composition Ab x Am,, and a little angite and 

 magnetite. One of the coarse-grained segregations, or simple crystalli- 

 zations, consists of labradorite and hornblende, in allotriomorphic crystals 

 1 mm. in diameter, with magnetite and apatite in irregularly shaped grains. 

 The feldspar and hornblende have the same characters as the phenocrysts 

 in the surrounding andesite, and there can be no doubt as to their being 

 crystallizations from the same magma. The structure is granitic. This 

 sheet is cut by two dikes of hornblende-andesite (1565, 1566), 15 and 20 

 feet wide, that trend northwest. They are rather dense and slig'htly vesic- 

 ular, with flattened cavities, are light gray, and carry scattered pheno- 

 crysts of hornblende and few feldspars. The groundmass is pilotaxitic 

 to hyalopilitic in one case, and holocrystalline with prismatic and tabular 

 feldspar microlites in the other. The phenocrysts are like those in the 

 andesites just described. South of the inclined sheet are two small dikes 

 trending northwest. They are hornblende-andesite, similar to the last. 

 Another parallel dike, 1 5 feet wide, consists of gray hornblende-andesite 

 (1568) crowded with minute phenocrysts, some of the hornblendes being 

 prominent. The rock is dense, and is black and glassy near the contact wall 

 (1567). It cracks into small prisms. The groundmass of the center of the 

 dike is hyalopilitic, and at the margin is a beautiful brown glass with abun- 

 dant microlites of feldspar, pyroxene, and magnetite. The phenocrysts are 

 labradorite, greenish-brown hornblende, hypersthene, and less augite, with 

 magnetite. 



Another dike, with the same northwest trend, consists of several vertical 

 sheets, from 2 or 3 to 4 or 5 feet thick, inclosing wedges of breccia. It is 

 dark bluish gray, compact, without phenocrysts (1569), and proves to be a 

 basalt rich in olivine. It exhibits fine prismatic cracking in curved columns 

 perpendicular to the walls of the dike, and is in part thinly fissile parallel 

 to the walls, and in places is fissile across the columns. The rock is dense and 

 glassy near the contact, and consists of an aggregate of prismatic feldspar 

 microlites and more abundant grains of augite and magnetite, with almost 

 microscopic phenocrysts of serpentinized olivine. 



Some distance south of the dikes just noted is a dike of dense gray 

 rock without megascopic phenocrysts (1570, 1571). It is a homblende- 

 pyroxene-andesite, and is glassy and black near the contact face. The dike 

 is from 10 inches to 3 feet wide and trends northwest. The groundmass of 



