18() IL V. (TSHIXlr — Ar<,ITK-SYEXITE (JXKISS NKAK LOON LAKE 



a lon^ ixn-iod hefbre the deposition of the Potsdam sandstone the Adiron- 

 dack region was above sealevel, so that none l>ut igneous rocks are found 

 representing the time interval between tlie Potsdam and the only older 

 sedimentary formation known in the district, the crystalline limestones 

 and associated qiiartzites and gneisses. These latter are evidently the 

 equivalents of the Grenville series of Canada. The syenites are younger 

 than the Grenville rocks, for they cut or include them, as already noted. 

 On the other hand, they are older than the youngest of the pre-Potsdam 

 rocks, the diabases, for tliey are cut by them. In the first cut, 100 rods 

 south of the depot at Loon lake, the syenite is traversed by a diabase 

 dike o ieei wide. 



These diabases have not ])een metamorphosed, wdiereas the syenites 

 have suffered change of such a character as to indicate that during the 

 process they were deeply enough buried beneath deposits since eroded 

 away to be in the zone of flow", so that a long time interval must lie 

 between the two. In addition to these syenites, the gabbro rocks and 

 certain granites are later than the Grenville rocks and much older than 

 the diabases. The relationships of the gabbros, syenites, and granites 

 to one another wnll be reverted to later. It should be stated that they 

 are older than the Essex county, Massachusetts, rocks, wd)ich cut Lower 

 Cambrian strata, according to 8ears,'^ and are likely older than the 

 ]\Iount Ascutney syenite as w^ell. 



Adirondack Syenite Arkas 



loon lake 



The Loon Lake syenite belt is quite extensive, having a length of 

 nearly 20 miles and a breadth of 10, though of irregular shape. These 

 figures are advanced with some hesitation on account of the difficulty 

 of recognizing the rock in ordinary exposures, especiahy tow^ard the 

 periphery of the belt. It is only in recent cuts that fresh material is to 

 be obtained. In ordinary outcrops a rusty, brown gneiss prevails, wdiich 

 may or may not shoAv greenish, less weathered nodules when broken. 

 The much elongated character of the quartz- augen often shows character- 

 istically in these weathered rocks, and considerable dependence has been 

 placed upon it as k criterion for their recognition ; but this is only of 

 avail in the more acid phases, whereas the fresh rocks are found to pass 

 into varieties in wdiich the ferromagnesian silicates become more promi- 

 nent and quartz recedes. Weathered rocks of this type have a wide 

 range. They are finer grained and better foliated than the type and, 



*J, H. Sears : Bulletin Essex Institute, vol. XXII, 1890. 



