Cusiiixg — Potsdam Boundary. 



11 



sedimentary origin, yet has been so profoundly modified that practically all 

 trace of clastic structure lias disappeared. The larger part of the rocks have 

 a very finely granulitic structure, having undergone nearly complete re-crystal- 

 lization. The dynamic metamorphism to which they have been subjected has 

 given them a foliation in common with the older gneisses, rendering the field 

 relations of the two exceedingly obscure. 



From Parishville westward to Potsdam and beyond, the Grenville series 

 comes to the Potsdam boundary and may be seen to great advantage. Here, 

 on the western side of the Adirondacks, it differs somewhat in character from 

 the similar rocks to the eastward, being more widely distributed, less faidted, 

 less completely metamorphosed, hence with its original sedimentary character 

 less disguised. The distance from the great anorthosite intrusion, which has 

 so profoundly affected these rocks on the east, is a probable cause for these 

 differences. 



That the eastern and western representatives are equivalent seems to be 

 beyond question. It is, however, quite desirable that they should be connected 

 by tracing them across Franklin county. 



III. The Anorthosite Intrusion. At some time after the deposition of 

 the rocks of the Grenville series, a great batholitic mass of the highly felds- 

 pathic variety of gabbro known as anorthosite was intruded into the existing 

 rocks. The structure of the gabbro indicates that it solidified at considerable 

 depth, hence the rocks with which it is now in contact must have been buried 

 beneath other rocks, since wholly removed by erosion. The anorthosite has 

 its largest development in Essex county. In Clinton county it is exposed 

 around Keeseville and on Catamount mountain and Rand's hill. It occurs in 

 the eastern part of Franklin county but its extent is not known. Further 

 westward its presence is problematical. 



IV. Later Gabbros. Dark colored rocks of the gabbro family, of greater 

 basicity than the anorthosites, occur wide-spread in the Adirondack region, 

 extending far beyond the limits of the anorthosites. They occur most 

 frequently in the form of dikes or sheets of no very great width, though 

 larger masses are not uncommon In part they are certainly later than the 

 anorthosites, for they cut them. They may in part represent apophysae from 

 them, and basic peripheral portions of the intrusion, though this has not yet 

 been demonstrated. The gabbro at Port Henry has been described in detail 

 by Kemp,* and some occurrences from the western Adirondacks descril»ed by 



•J. F. Kemp, Bull. G. S. A., Vol. V., pp. 213-224. 



