486 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



least four dikes exposed here, cutting quartzitic rocks of the Prichard 

 formation, tliree of whicli are about 4 feet in average width. Thesa 

 three dikes, while completely decomposed, resemble the dark 

 spessartites of the Bailey's Pond side. The fourth, which is the 

 westernmost, is somewhat fresher in appearance. At the level of 

 the road it is about 8 feet wide, but it narrows upward. On the 

 surface it is traceable for about one-fourth of a mile in a north- 

 westerly direction, and may connect with dikes in Italian Gulch, 

 which are in the line of strike. 



In the hand specimen this rock is fine-grained and of a greenish- 

 gray color. The most abundant megascopically distinguishable 

 mineral is micaceous and of a greenish color. Small white grains, 

 presumably feldspar, are visible under a lens. The rock is tena- 

 cious but has a tendency to cleave parallel to the walls of the dike. 



Under the microscope the rock is equigranular except for rare 

 phenocrysts now consisting entirely of serpentine which is clearly 

 secondary after olivine. The other original minerals are colorless 

 augite, biotite, and orthoclase with accessory iron ore and apatite. 

 The rock is very much altered. Among the secondary minerals the 

 most abundant is chlorite, which has developed at the expense of the 

 original biotite. Calcite in infiltrated patches and secondary quartz 

 are abundant. Epidote occurs in places as an alteration product of 

 biotite and sericite is common in the feldspar. The olivine pheno- 

 crysts are now represented by masses of serpentine, dotted with iron 

 ore which are commonly surrounded by a rim of biotite and augite. 



The pyroxene, which is colorless and unaltered in thin section, 

 is near diopside in composition, and is perhaps the most abundant 

 mineral of the rock. It forms prismatic individuals with poorly 

 developed form and barely perceptible cleavage. Biotite occurs 

 in thin laths and tablets, pleochroic in tones of dark and light brown. 

 There now remain only ragged remnants of the original biotite sur- 

 rounded by chlorite. Tlie orthoclase, which is greatly sericitized, 

 forms poorly individualized interlocking grains. Iron ore is abun- 

 dant in small grains and apatite occurs in the usual prismatic crys- 

 tals. Quartz is abundant in the interstices, but it is all clearly 

 secondary. The rock thus has no features in common with the spes- 

 sartite of the Bailey's Pond side. 



Augite Minette, Murray Hill. — Another rock, collected by Doctor 

 Umpleby and examined only in thin section. Contains an unusual 

 number of sharp euhedral crystals of augite in a groundmass com- 

 posed of biotite and orthoclase. The augite is for the most part 

 clear and colorless but the crystals have at times a narrow faintly 

 pleochroic j)ale-green border of aegirite-augite. The orthoclase is 

 of the peculiar type composed of bundles of prismatic fibers. It is 



