MKTAMORPHISM OF THE LIMESTONES AS A WHOLE. 281 



conclusion is correct nepheline should probably be added to the list of 

 contiict-minerals, for the points where it occurs are ver}'' close to the con- 

 tact, thouf]jh not absolutely on it. It must be said, however, against 

 such a supposition that there has been found neither gieseckite nor 

 nepheline at the actual contacts examined. The question as to the 

 origin of these minerals is thus still open, witli the probabilities as just 

 stated, Init of the presence of contact-motanior[)hism there is not the 

 slightest doul)t. 



Metamorphisin of the Limestones as a Whole. — This brief consideration of 

 contact-action in the limestones brings up the question as to how much 

 of the general mctamorphism of the limestone series has resulted from the 

 intrusion of the great gabbro masses. That the cr3^stallization of the 

 rocks is a result of contact action on a large scale was first suggested by 

 Van Hise,* and tlie idea has been referred to l)y Professor Kern}) in the 

 preceding paper. In Essex county, where the limestone occurs in iso- 

 lated patches completely surrounded by gabbro, the inference seems 

 quite justitial)le; but great caution should be used in ai)plying this ex- 

 planation to the region here considered, where the conditions are very 

 different, as shown in the foregoing. Instead of small patches, we have 

 extensive areas of limestone, while the rocks of known intrusive nature 

 are quite limited in area as compared with Essex county. So far as 

 examination has l)een made, the limestone is thoroughly crystalline 

 throughout, tlie degree of crystallization not depending upon the position 

 in the belt nor the proximity of intrusives, except in the case of a narrow 

 zone in close contact with the latter. If the mctamorphism were caused 

 by the intrusion we should expect to find a different state of affairs, a 

 more complete crystallization in the neighborhood of the igneous rock ; 

 but even did such a relation exist it would seem very doubtful whether 

 the igneous rock present would afford a suflicient cause for the complete 

 crystallization of such extensive areas of limestone and iml)edded gneiss. 

 Should future investigation, however, prove a large percentage of the 

 ma.ssive gneisses to be of intrusive nature, this difficulty would be con- 

 siderably reduced. When the actual contact-zones are considered, their 

 narrowness and sharpness of definition are striking. Instead of a gradual 

 increase in crystalHzation and number of different minerals as the ign-eous 

 rock is ai)proaclied' tlicre is no ])ercei)tible change till within a few feet 

 or inches of the latter, and then there is a distinct zone of ccjutact- 

 producta. Were the intrusion the cause of general mctamorphism it 

 would seem that the contact-zone would be much wider and wouM shade 

 gradually \nU) the ordinary limestone. 



In this connection mudi interest attaclies to an isolated patch of lime- 



• Bulletin 88, U. 8. Oeol. Survey, p. 399. 



