l8 NEW YORK STATE MUSEUM 



in table 2, more or less readily removed from the crushed rock by 

 washing. 



Johnstown district. Samples taken from the Potsdam outlier 

 in the Mohawk valley are somewhat similar to the rock from the 

 Moira-Bangor area, but very much less sericitic and more feld- 

 spathic; the feldspar grains are all more or less altered, most of 

 them having been kaolinized to a greater or less degree. 



The feldspathic, rather than sericitic, nature of the impurities, 

 and a slightly smaller average grain size, constitute the chief differ- 

 ence between the Potsdam in this locality and that in Franklin and 

 Clinton counties. An unusual and interesting feature in connection 

 with the feldspar is that in almost every instance each rounded and 

 altered feldspar grain has, like the quartz, been enlarged by second- 

 ary growth; the later and added on feldspar has been altered to 

 some extent also, but as a rule not to such a degree as the older 

 original and rounded grains, so that the evidence of secondary 

 enlargement is clear and unmistakable. 



The quartz contains liquid and gas inclusions and much minute, 

 indeterminate dusty matter, as well as small zircons, minute rounded 

 grains of tourmaline, rutile in needles and stouter crystals as well, 

 small groups of hematite specks, small needles of sillimanite and 

 other very minute indeterminable crystals. 



Larger grains of similar sorts occur interstitially to some extent, 

 and in the less pure beds a little more iron oxide. 



The sum total of all these impurities is not great, however, as 

 may be seen by referring to the analysis, table I. 



Quartzite phase at Fort Ann. The rock in the vicinity of Fort 

 Ann, Washington county, is in general too highly feldspathic to 

 be of much value. Figure 15 illustrates the general character and 

 habit of this rock; the grains are smaller than those of the sac- 

 charoidal sandstones just described, and the rock is thoroughly 

 indurated. Silicification and recrystallization have not entirely 

 obliterated all traces of former structure, however; in occasional 

 grains the former rounded character is still plainly evident. 



The grains are angular, fairly well interlocked, and reasonably 

 free from much included matter. The chief objection to the rock 

 is the presence of considerable feldspar. 



In grain size, structure and lithologic character the rock resembles 

 some phases of the Poughquag. Small rounded grains of tourma- 

 line, minute zircon crystals, rutile needles, an occasional minute 

 patch of leucoxene, small black metallics resembling magnetite, and 



