BUILDING STONES. 189 



General conclusions.] 



of aggregation of the individual grains and their cementing bond, as well 

 as on the chemical relation of the silica to the other chemical ingredients. 

 For instance, a nearly pure arenaceous rock of quartz, like the St. Peter 

 sandstone, containing about ninety-eight per cent, of quartz, is still so fra- 

 gile that it is easily excavated with the fingers alone, there being no suffi- 

 cient cementing material to bind the grains to each other. The same kind 

 of sandstone, however, with a sufficient cement, makes a very fine build- 

 ing material, as the stone at Hinckley in Pine county, No."" 30. If 

 furthermore there be a partial chemical union between the silica and the 

 other elements, as in the Pipestone quartzyte (No. 12), the stone is rendered 

 more durable still, and more firm under pressure. And if the chemical 

 union be complete, so as to result in a perfect crystallization of the rock, 

 the stone becomes one of the granites or syenites and necessarily takes the 

 highest rank in the comparative scale of values. 



A stone that has a high percentage of alumina (if it be also non-crystal- 

 line), or of organic matter, or of protoxide of iron, will generally dis- 

 integrate rapidly. Such stones generally are of a bluish color. 



A stone of rather poor quality by reason of a weak physical structure 

 or an unfortunate chemical composition, is improved by the absolution of iron. 

 The sesquioxide of iron furnishes a bond to hold the grains together, in case 

 of an originally weak cement, and it takes the place of the protoxide of iron, 

 or of organic impurities. 



A stone of high grade, having a crystalline texture, or a superior chem- 

 ical constitution, is impaired by the absorption of iron through the process of 

 weathering. The first effect of weathering in granites is visible in the loss 

 of transparency of the individual grains either of feldspar or of some of 

 the other minerals, the result of the penetration of ferruginous matters 

 within them. A dolomite which is changed in color by this means is also 

 weakened. This is apparent on comparing No. 19 with No. 23, or No. 38 

 with No. 40. 



A stone that has no very weak element in the list of its credits, will be found 

 more valuable than one that reaches an average result perhaps higher, but 

 has one or more very low points on the scale of credits. The ultimate use- 

 fulness of a stone in average construction, depends on its weakest element. 



Durability, texture, color. These are the essential qualities to be consid- 



