50 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 66. 



The adjacent rock for an average distance of about 5 mm. is visi- 

 bly altered, being somewhat bleached in appearance. In powder 

 under the microscope the feldspars of this bleached zone are found 

 to be filled with a fine muddy dust but show none of the coarse 

 flaky sericite, nor are they albitized. The pyroxene is altered to 

 colorless diopside which shows a grating structure, probably a pseudo- 

 morphous remnant of the cleavage cracks and basal parting of the 

 original augite. 



ALTERATION OF NORMAL DIABASE ADJACENT TO ZEOLITE-BEARING VEINS. 



The rock of a number of zeolite specimens from shear zones which 

 contained cavities filled with minerals, including prehnite, apophyl- 

 lite, laumontite, etc., was examined to determine the extent and 

 character of the hydro thermal alteration accompanying the filling of 

 the zeolite veins. In the hand specimen, in contrast with the nor- 

 mal rock, this has a peculiar dead lack of luster and harsh dry feel, 

 the feldspars are white and opaque-looking, and the dark minerals 

 have a brownish to pistachio tinge. Sections were cut from the 

 most intensely altered portions and under the microscope these were 

 found to have suffered some alteration but no drastic mineralogical 

 changes. The feldspars are extensively dusted with minute flakes 

 of sericite, but have not been otherwise altered. The principal action 

 on the augites has been the introduction of much finely flaky, yellow- 

 ish gi-een material, probably serpentine or a chlorite, into some 

 crystals, while adjacent crystals are perfectly fresh. Iron ore is 

 unchanged showing none of the replacement by titanite which marks 

 the earlier hydrothermal alteration along seams. 



A specimen from the strong shear zone at the south end of the 

 quarry, where the dip of the joint system changes from west to east, 

 had about the same microscopic characters. 



These later vein-filling solutions apparently were lower in tempera- 

 ture and lacked the vigor of the earlier emanations which produced 

 the preceding types of alteration, 



"DIABANTITE varnish" on SLICKENSIDED JOINTS. 



The fact that the joints of the north-south systems are uniformly 

 coated with a lustrous coating of slickensided black chlorite has already 

 been noted. This forms layers up to 2 mm. in thickness having a 

 smooth-polished, grooved-fibrous, lumpy, or rugose structure. These 

 are entirely similar to coatings occuring in the Triassic traps through- 

 out the Newark series. I have specially noted their occurrence in 

 the quarries of the Westfield, Mass., where they are abundant. 



When these layers are crushed and examined microscopically they 

 are found to consist very largely of chlorite, although there is always 

 some ground-up diabase incorporated with the chlorite, and for this 

 reason the material was not suited for analysis. 



