PROCEEDINGS OF THE NATIONAL MUSEUM. vol. G6. 



8. Hydrothermal joint and cavity fillings- Continued 

 h. Minerals. ^ — Continued. 



The diabase exposed in the walls of the quarry is very strongly 

 dissected in what seems, at first glance, to be an intricate complex 

 of joints and fractures. More thorough study resolves this fractur- 

 ing into two relatively simple systems. For convenience of descrip- 

 tion and reference these have been designated the north-south joint 

 system and the east-west fissures. 



The north-south joint system is a series of closely spaced, steeply 

 dipping joints which slice the diabase into slabs of the magnitude of 

 50 cm. in thickness, the variation being from 10 cm. to 100 cm. 

 At the southwest corner of the quarry these joints, which are very 

 pronounced, have an average strike of N. 15° E. and dip 60° to 65° 

 east. At the northwest corner these joints, which are not nearly so 

 well defined or so well exposed, seem to strike about N. 15° W. and 

 to dip east at 85°. In the main face of the quarry, which is more 

 or less parallel with the strike of this joint system, they vary in strike 

 from about N. 15° E. to N. 30° E. and dip from 70° to 80° west. 

 The change from east to west dip in this joint system is not gradual 

 but in the south quarry wall is seen to take place suddenly at a 

 fissure marked by a considerable zone of shearing. This break seems 

 to extend in a north-south direction across the floor of the quarry 

 and probably intersects the north wall in a notch marked by unusu- 

 ally deep weathering. 



The joints are almost invariably coated by a black shining and 

 slickensided veneer of the chlorite here referred to diabantite. This 

 coating, which may be lumpy, fibrous or grooved, is usually from one 

 to five millimeters thick and consists of the chlorite in relatively pure 

 form. There is relatively little crushing along the north-south direc- 

 tion, although occasionally a joint thickens into a zone of sheared 

 rock from 5 to 30 cm. wide. One such streak of sheared rock in the 



