j,.RT. 2. PETROLOGY AT GOOSE CREEK — SHANNON. 7 



southwest corner of the quarry was seen to containdiabantite-coated 

 fragments with interstitial apophylHte, datoHte, and calcite; and one 

 m the northeast corner furnished calcite, datolite, prehnite, laumontite, 

 and stilbite. The rock adjacent to these joints is for the most part 

 fresh and free from indications of high temperature hydrothermal 

 alteration. It is not beheved that any single north-south joint has 

 been the locus of any great movement. However, such a closely 

 spaced joint system, where each fracture contributed its share to the 

 total, might have resulted in a large aggregate movement, and this 

 would be aided by the diabantite which would serve as a lubricant to 

 facilitate slipping with a minimum of crushing. The general slickensid- 

 ing of the susceptible diabantite may well be the result of a long 

 series of relatively shght creepings of adjacent blocks. 



What is here designated the east- west fissure system consists of a 

 number of somewhat irregular or curved fractures varying in strike 

 from N. 30° W. to N. 75° W. with a dip of from 75° to 85° northeast. 

 These are not close spaced enough to be designated a joint system, 

 well marked breaks being separated by an average distance of 10 me- 

 ters. There are many features which differentiate these fractures from 

 the north-south joints, even when, as sometimes happens, the two 

 •coincide approximately in strike. The east-west fissures have more 

 the appearance of sharp breaks and are more frequently accompanied 

 by crushed and sheared zones. They are not notably slickensided, 

 mainly because they are not coated with the easily polished diaban- 

 tite. The coating on the surfaces is mostly light colored in tones of 

 gray to blue-green or gray-green, and consists in the main of finely 

 fibrous hornblende or of a light gray-green chlorite which contrasts 

 sharply with the glossy black of the diabantite varnish on the north- 

 south joints. All of the aplites seen in place occupied or were par- 

 allel to the east-west fractures and in some places coarse " pegmatite " 

 phases seemed somewhat aligned in this direction. Moreover the 

 east-west system seems characterized by more hydrothermal alter- 

 ation contiguous to the joints. The various later secondary hydro- 

 thermal vein minerals, including prehnite, datolite, apophyllite, and 

 zeolites appear to be developed along shears in this direction some- 

 what more frequently than along the north-south joints. Although 

 most of the specimens of such minerals which were studied came 

 from broken piles of rock, the system of fractures in which they 

 occurred was indicated in the altered nature of the inclosing diabase, 

 the material from the north-south system being much darkened by 

 diabantite coatings, while the hydrothermal alteration of rock 

 from the east-west fissures has resulted in a bleached and chalky 

 appearance. 



The relative age of the two systems above considered is not estab- 

 lished entirely. There is, as has been mentioned, an occasional 



