28 NEW YORK STATE MUSEUM 



reality a marginal part of the same intrusion, or a separate in- 

 trusion.^ The porphyritic border phases of the rock as they occur 

 about Saratoga, where the relation to the main rock is certain, 

 are so precisely like this porphyritic rock from Alexandria as to 

 leave no doubt in our mind that this is also a border phase of 

 the syenite intrusion. Such border phases of the rock, of por- 

 phyritic nature, are proving a quite normal feature of the syenite 

 of the region. 



The rock here varies about as it does elsewhere except that the 

 more basic phases are lacking. Granitic varieties, however, are 

 abundant. This seems to us likely due to the fact that the sur- 

 rounding rocks in the quadrangle are mostly of siliceous nature. 

 Much more than in the case of the other intrusives of the region 

 there is found for the syenite a usual relation between the nature 

 of its border phase and the character of the neighboring rock ; 

 such a relation as indicated by Kemp for the occurrences of the 

 Elizabethtown quadrangle, and by the writer for the Long Lake 

 quadrangle.^ 



But one variety of syenite on the quadrangle seems sufficiently 

 novel to merit special notice. In the quarry by the Hudson near 

 Spiers dam, is a very gneissoid dark green variety of syenite, one 

 of the many varieties occurring there, which is really not a syenite 

 at all, but diorite, 80 per cent of its feldspar being andesine (about 

 AbgAn^). Pyroxene, hornblende, biotite, magnetite, apatite and 

 zircon form about 15 per cent of the rock, some 7 per cent is 

 quartz, and the remainder feldspar. The excellent exhibit of the 

 rock shown in the quarry shows rapid and great range in com- 

 position, bands of granite, syenite and diorite appearing. All are 

 plainly varieties of the one rock type. The material is almost pre- 

 cisely like the syenite at Little Falls in appearance and in variation, 

 except that at Little Falls the variation between extreme types is 

 not usually so rapid. In these very gneissoid syenites of the east, 

 garnet is a more common mineral than in the mid-Adirondacks or 

 on the west. There it is usually confined to the basic, border 

 phases, or to the narrow dikes, whereas here it comes quite fre- 

 quently as a constituent of the normal and the acid varieties. 



Diabase dikes of late Precambric age. Nearly everywhere in 

 the Adirondacks all the other Precambric rocks are found cut by 



1 N. Y. State Mus. Bui. 145. p. 39-40, 182-84. 



2N. Y. State Mns. Bui. 138, p. 81; N. Y. State Mus. Mul. 115. p. 478-79. 



