GEOLOGY OF OGDEXSBURG I9 



A narrow belt of Grenville gneiss margins the jNIacomb bathylith 

 on the south and just beyond appears the northern edge of a mass 

 of granite, which runs widely to the south on the Gouverneur sheet 

 and which must be studied there. It is a coarser granite than the 

 other, holds but few inclusions, and quite probably belongs to the 

 later intrusives which we have mapped as syenite. Its precise rela- 

 tions must be determined by work on the Gouverneur area. 



The Dekalb granite, which occupies the southeast portion of the 

 Ogdensburg sheet, extends to the northeast onto the Canton quad- 

 rangle, where it has been mapped and studied by ]\Iartin, and also 

 runs southwest for an unknown distance on Gouverneur. Its full 

 breadth of about 4 miles is shown, as Grenville rocks adjoin it to 

 the southeast just at the sheet corner. 



In so far as most of the granite is concerned it is a fine-grained, 

 red orthogneiss, quite like that at Macomb and Alexandria, chiefly 

 a feldspar-quartz rock, with very little mica, and with inclusions of 

 amphibolite solely, so that we should have little hesitation in corre- 

 lating it also with the Laurentian, except for two things. At the 

 western edge it becomes fairly coarse grained and, though quite 

 gneissoid, is a quite different looking rock from the ordinary ortho- 

 gneiss. At the extreme southeast outcrop on the sheet also the 

 rock is of different character, much more micaceous than normal 

 and resembling a mashed porphyry; in other words, bearing some 

 resemblance to the porphyritic granites here mapped as syenite and 

 regarded as belonging unquestionably to the later group of intru- 

 sions. One of the peculiar things about the Precambrian geology 

 of the Ogdensburg sheet is the manner in which these long tongues 

 of porphyritic granite appear cutting the Grenville rocks, but, so 

 far as observed, never invading the adjacent granite-gneiss bodies. 

 It is one of the problems on which w^ hope to get some light 

 from the study of the wider exposures on the Gouverneur sheet to 

 the south. We are in doubt as to the age of this particular granite, 

 as to whether it should be classed with the earlier or the later 

 group of intrusions. 



The bulk of the Dekalb granite consists of finely granular 

 orthogneiss, composed almost wholly of quartz and feldspar, and 

 very similar to the Macomb and Alexandria rocks. Along its west- 

 ern margin, where it is coarse, it directly adjoins a belt of solid 

 Grenville limestone, yet it remains of red color and shows no indica- 

 tion of the bleaching to white which the general Laurentian granites 

 show in like situation. Nor does it show any bleaching near the 



