THE MINING AND QUARRY INDUSTRY I915 27 



It is essential, of course, that the feldspar should have a high 

 content in potash. Of the varieties which occur in nature only 

 two fill this requirement; they are orthoclase and microcline, each 

 having the same chemical composition — silica 64.7 per cent, 

 alumina 18.4 per cent and potash 16.9 per cent. As a matter of 

 fact, the theoretical percentage of potash as given is never attained, 

 since there is always some soda present as a substitute. It is not 

 uncommon, furthermore, to find the potash feldspar intergrown 

 with albite or soda feldspar so as to reduce the amount of potash 

 in the material very largely. In a quarry- way, 10 to 12 per cent 

 of potash is about the maximum that can be expected, and this 

 only under the best conditions with the aid of more or less sorting 

 and cobbing for the removal of waste. 



It is the writer's experience that pegmatite bodies which will yield 

 a uniform content of 10 per cent potash are extremely uncommon. 

 Not only is the content likely to be reduced by admixture with 

 other sorts of feldspar, but there is always a considerable percentage 

 of quartz and iron silicates which has to be reckoned with, the 

 amount varying with each locality and to a greater or lesser extent 

 in different parts of the same body. Pegmatite is very prone to 

 variation as can be seen in nearly every occurrence that is 

 sufficiently well exposed to afford an estimate of its character. A 

 pegmatite that will yield 75 per cent of potash spar quarrywise is 

 exceptional, at least among the explored bodies of this State. 



Pegmatites of such dimensions that they will afford the neces- 

 sary quantity of material to justify the expense of establishing a 

 works are not. very abundant. An available supply of several 

 million tons probably would be required, since the first cost of 

 plants is high and the capacity must be large to afford the necessary 

 margin of profit on the output. It is desirable also that the quarry 

 should be convenient of access, with low freight rates on fuel, and 

 with a supply of limestone nearby. Altogether the conditions are 

 very definite and exclusive, much more difficult to meet than most 

 writers on the subject seem to have realized. 



The feldspar resources of New York have been recently investi- 

 gated and described in detail. 1 Most of the local pegmatites that are 

 of any considerable size are found among the crystalline schists, 

 gneisses and acid igneous rocks of the Highlands and Adirondacks. 

 They occur as dikes or tabular bodies which intersect the country 

 rocks ; the largest ones, however, have the form of bosses and 



1 The Quarry Materials of New York — Granite, Gneiss, Trap and Marble. 

 N. Y. State Mus, Bui, j8i, p, 154-75. 1916. 



