28 NEW YORK STATE MUSEUM 



The Poughquag quartzite near Peekskill, in Westchester county, 

 is not of sufficient average purity, although it possesses the proper 

 structure. 



The manufacture of tube mill liners and pebbles. Rock used 

 in the manufacture of linings and grinding pebbles for tube mills 

 should be very hard and tough ; these characteristics are more essen- 

 tial than great purity. 



Most quartzites possess the requisite hardness, but some are so 

 brittle that their toughness is relatively low as compared with rocks 

 having very high toughness, like diabase; in spite of this disad- 

 vantage, quartzites are being used very successfully for this purpose, 

 and their use is increasing. The writer is indebted to the New York 

 State Highway Commission, bureau of tests, for hardness and 

 toughness tests, made on various samples taken from the different 

 formations studied; the results of these tests appear in table 4. 

 According to these tests the hardness is satisfactory in all cases, 

 but the toughness is low. The actual behavior of these rocks when 

 crushed in a mortar, does not accord very well with the results listed 

 for toughness ; for example, the white saccharoidal variety of the 

 Potsdam sandstone from the vicinity of Johnstown, crushes readily 

 to a granular sand, whereas the Poughquag quartzite, and the 

 Shawangunk conglomeratic quartzites crush with much greater 

 difficulty. 



Table 4 



Tests made in the testing laboratories of the bureau of tests, New York 

 State Highway Commission. 



Notwithstanding this behavior, the toughness of the Potsdam 

 sandstone, as determined by tests, is 6.0; that of the Shawangunk 

 (Accord, no. 9) likewise 6.0, while a sample of the Poughquag 

 quartzite from Peekskill is but 5.0; this is surprising in view of the 

 firmly interlocking habit shown by the Poughquag quartzite (see 

 figures 2 and 3), and, to a less extent, by the Shawangunk as well 

 (see figures 8 and 9). 



