Cusiiing — Geology of Clinton Count r. 



547 



show the rude ophitic structure already described. The larger feldspars often 

 show beautiful pressure effects, the crystals not only being broken but 

 frequently bent. In one slide is a well-twinned crystal, about seven 

 millimetres long, in which the gradual variation in extinction from one end of 

 the crystal to the other amounts to 25°, showing a bending to that amount, 

 yet with no apparent breakage of the crystal. 



The contacts between these basic bands and the anorthosite are often as 

 sharp and clear as intrusive contacts, so that slides may readily be prepared 

 consisting half of one and half of the other. 



As has been already stated, these basic gabbros are so absolutely like the 

 basic gabbroic bands found in the gneisses, that there can be no question as 

 to the identity of the two. Neither in the hand specimen nor in thin section 

 can they be distinguished from each other. Both the granular and the 

 ophitic phases occur in both situations. It would seem most probable that 

 they are to be regarded as basic segregations from the main anorthosite 

 intrusion, which have been stretched out into bands parallel with the foliation 

 of the enclosing rock as a result of dynamic metamorphism. Such 

 segregation would take place mostly toward the periphery of the mass, and 

 such portions as were squeezed out into cracks in the enclosing rocks would 

 be mainly of this type. But on the other hand the frequent sharp boundaries 

 between the basic gabbro and the anorthosite, with no sign of gradation into 

 each other, and the much wider distribution of the gabbro, can not but give 

 rise to the suspicion that, at least in part, the gabbro intrusion may have been 

 subsequent to that of the anorthosite.* 



A massive band of basic gabbro which is found in the anorthosite at a 

 point a little over a mile west of Keeseville, just north of the river road, is 

 interesting as showing a gradation toward diorite. Monoclinic pyroxene and 

 garnet are found in the rock but are very subordinate to the hornblende, 

 which shows great development, while hypersthene is absent. A hint of this 

 is given in the appearance of the rock, which is more schistose than the 

 normal gabbro. The main interest attaching to the occurrence is from its 

 possible bearing on the origin of the hornblende gneisses of the gneiss series, 

 which the rock much resembles. 



Two miles west of Keeseville, on the river bank, is a knob of gabbro 

 which is interesting as indicating a transition to gneiss. It is of brown color, 

 finely granular, and contains quite a little quartz and microperthitic 



•Later work by both Professor Kemp and the writer demonstrates that much, if not all of the basic gabbro is of later date 

 than the anorthosite, as indicated here al Keeseville. 



