186 H. v. CUSHING AU(irrK-SYEXITIO GNEISS NEAK LOON LAKE 



a lono- poriod l)e(oi"e tlie deiiosition of the Potsdam sandstone the Adiron- 

 (L\ck region was above sealevel, so tliat none l)ut igneous rocks are found 

 re])resenting tlie time interval l)etween the Potsdam and tlie only older 

 sedimentar}' formation known in the district, the cr3'stal]ine limestones 

 and associated quartzites and gneisses. These latter are evidently the 

 equivalents of the Grenville series of Canada. Tlie syenites are 3'ounger 

 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 they are cut by them. In the first cut, 100 rods 

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

 dike 3 feet wide. 



These diabases have not l)een metamori)hosed, whereas the S3'enites 

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

 process they were deeph' 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. Jn 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 gal)bros, S3'enites, and granites 

 to one another will be reverted to later. It should be stated that the3^ 

 are older than the Essex count3', Massachusetts, rocks, which cut Low'er 

 Cambrian strata, according to Sears,''^ and are likel3^ older than the 

 Mount Ascutne3' S3'enite as well. 



Adirondack Syenite Areas 



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 difhcult3" 

 ot recognizing the rock in ordinar3^ exposures, especiall3^ toward the 

 peripher3^ of the belt. It is only in recent cuts that fresh material is to 

 be obtained. In ordinar3^ outcro])s a rust3', brown gneiss prevails, which 

 may or may not show greenish, less w'eathered nodules Avhen broken. 

 The much elongated character ot the quartzaugen often shows character- 

 isticall3' in these weathered rocks, and considerable de})endence has been 

 l)laced upon it as a criterion for their recognition ; but this is onl3^ of 

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

 into varieties in which the ferromagnesian silicates become more promi- 

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

 range. The3' are finer grained and better foliated than the t3^pe and, 



*.J. II. Sears : Bulletin Essex Institute, vol. XXII, IS'JO. 



