20 NEW YORK STATE MUSEUM 



long limestone tongue which lies within the granite. The coarser 

 phase of the rock is that portion which lies between these two 

 limestone belts, and it may be a wholly different mass from the 

 orthogneiss which lies east of it. In our experience with the two 

 granites of the region, the older or Laurentian is the one which 

 loses its red color at limestone contacts, and we have not yet met 

 with the younger one affected in the same way. Nor, in our labora- 

 tory experiments on the question, did we succeed in bleaching it.^ 

 We are here simply trying to marshal the arguments for and 

 against an age classification of this granite, and, as stated, we are 

 in doubt. If it consists of two separate granites we should have 

 no hesitation in correlating the western portion with the later, and 

 the eastern portion with the earlier granite. But we have little 

 direct evidence of an age difference between the two masses. 



In the eastern portion of the mass we came upon evidence of 

 bleaching of granite adjoining amphibolite inclusions, evidence 

 chiefly obtained from a quarry opened near Dekalb Junction to 

 obtain material for use on the state road under construction. It 

 may be remarked that such granite gneiss makes about the poorest 

 possible rock for road making, being exceeded in badness among 

 the local rocks only by the Potsdam sandstone. In this quarry the 

 granite was bleached for a few inches around many of the amphib- 

 olite inclusions. This is not a common feature of granit'e-amphib- 

 olite contacts and suggests that these particular inclusions are 

 altered limestones. 



Syenite. The rock mapped as syenite and regarded as a member 

 of the later group of intrusives is, so far as its occurrence within 

 the Ogdensburg quadrangle is concerned, not a syenite at all but a 

 granite, and its classification is based solely on its lithologic char- 

 acter, as compared with rocks elsewhere in the Adirondacks. It is 

 a porphyritic granite of a peculiar and definite type, a rock which 

 is of fairly common occurrence in the Adirondack region and which, 

 in our experience, occurs only as a marginal, granitic phase of the 

 syenite bodies of the region. It is because of this that it is mapped 

 as syenite here, and separated from the other granites, and because 

 of this that it is regarded as belonging to the later set of intrusives. 

 In the region here the two sets are not in contact and there is no 

 direct evidence of their relationship to each other. 



These peculiar granites are always porphyritic and often coarsely 



1 N. Y. State Mus. Bui. 145, p. 177-80. 



