HIGH-GRADE SILICA MATERIALS 17 



is not of very great purity. Occasionally, however, a fairly pure 

 and white variety occurs, such as that in the vicinity of Fort Ann, 

 Washington county ; here a thin-bedded, fine-textured, hard quartz- 

 rock, dipping gently in an easterly direction lies on both sides of 

 the canal, well exposed in many places and easily accessible. 



Some beds are quite white; analyses gave 98.82 per cent SiO 2 

 in one case, and 9545 per cent in another, the chief impurity being 

 feldspar. 



The rock is of no value for glass making, but might be used for 

 tube mill liners and pebbles, and in the manufacture of 

 refractories. 



A small quarry, not now being operated, is situated about i]/2 

 miles north of Fort Ann, several hundred feet west of the tracks 

 of the Delaware and Hudson Railroad, and convenient thereto; 

 the sample giving the lower silica value was taken from this quarry. 



Lithologic character. Photomicrographs of Potsdam rock from 

 (.he several localities mentioned are shown in figures II to 15. 



Moira-B.angor district. The rock from the Moira-Bangor dis- 

 trict varies in grain size; samples from the vicinity of Moira average 

 about 0.50 mm in grain diameter, while samples taken near Bangor 

 average about 0.30 mm. All grains are enlarged by secondary 

 growth, but no complex interlocking has resulted from silicification ; 

 the rock breaks down readily, when crushed, into a granular sand, 

 whose individual grains are more or less angular because of second- 

 ary enlargement, notwithstanding the original rounded character 

 of the grains. The quartz grains are fairly free from included 

 matter, containing only trains of liquid and gas inclusions, minute 

 groups of tiny hematite specks, minute rutile needles, occasional 

 small zircons, minute apatite crystals, and other very small specks 

 and tiny crystals whose identity could not be established. The 

 rock from both Clinton and Franklin counties is inclined to be 

 sericitic; some of the samples giving lower silica values contained 

 appreciable amounts of sericite; figure 13 illustrates this type. Asso- 

 ciated with the sericite in some places, are many very small pris- 

 matic and tabular crystals with high relief and low birefringence, 

 responding to the optical tests for barite, and judged to be that 

 mineral (see table i). Patches of leucoxene, iron oxide, kaolinitic 

 and sericitic matter, small black metallics, altered feldspar, small 

 shapeless grains of titanite, and occasional rounded tourmaline 

 grains constitute the rest of the mineral matter other than quartz; 

 all these are interstitial with respect to the quartz, and, as shown 



