690 NEW YOKK STATE MUSEUM 



40^ and should preferably be less than 25^. A stone with more 

 than the latter amount has a tendency to expand slowly with 

 age, specially if it runs low in clay. A western ISTew York rock 

 with 37^ of carbonate of magnesia and less than 2^ of silica and 

 silicates is aii example of this fact. The Rosendale cements con- 

 tain only about 20^ of magnesian carbonate with 30^ of clay. 

 The amount of silica and silicates present is highly important, 

 for they influence the hydraulic properties of the limestone, and, 

 as said before, the silica should be in combination with the alum- 

 ina. This is determined partly from the quantity of alumina 

 contained in the stone. For example, two stones, one from 

 Akron (N. Y.) and the other from the Rosendale region, show 

 according to Richardson respectively 35^ and 29^ of insoluble 

 material; but, from the amount of alumina and iron present, we 

 can see that there is very little clay in the Akron stone, while 

 there is much of it in the Rosendale rock, the former having 

 only 4.84^ of alumina and irdn, while the other has 10^. The 

 Rosendale in consequence makes a very superior cement, while 

 the Akron shows the qualities resulting from a deficiency in clay 

 but an excess of magnesia. The effect of this deficiency in clay 

 is to form a cement which heats and sets too quickly, but an 

 excess of clay can also be injurious, as already stated. 



Sulfur, when found in the limestone, is generally in the form 

 of gypsum (sulfate of lime) or pyrites (iron sulfid) but both 

 of these substances are seldom present in sufficient amounts to 

 be injurious. They may occasionally become reduced in burn- 

 ing when combined with iron oxid to produce a green color. 

 Alkalis such as potash and soda are harmless unless present in 

 more than 2^; an excess of them makes the rock fusible, and 

 such material has to be rejected. The following alkali percent- 

 ages, in different natural rock cement, are given by Richardson. 



Milwaukee, Kfi .87 



Milwaukee, Ka.p 1 . 64 



Fort Scott, Kfl 7 



Fort Scott, l^sifi 1 . 33 



