28 NEW YORK STATE MUSEUM 



many occurrences of the Potsdam sandstone in this State. When 

 the secondary quartz is united with the grains to build them out 

 into interlocking crystals, as sometimes happens, the material is 

 the most durable of all constructional stones. 



Limestones are made up mainly of the calcareous skeletons of 

 organisms, though often so finely comminuted as to be unrecogniz- 

 able to the unaided eye. There is also more or less of secondary 

 calcite, derived by solution and redeposition of the lime, which 

 serves to fill up the interstices and the interiors of the organic 

 remains. The calcite shows crystalline character, but is not so 

 uniformly developed in rhombic particles as in the case of marbles. 

 Besides calcite, the double carbonate of lime and magnesia, or 

 dolomite, may be present in similar form. Through its increasing 

 participation, the magnesia may replace the lime up to 20 per cent 

 or so. 



Though calcite is quite soluble in rain water and groundwaters 

 which contain carbon dioxide, limestones, when compact and well 

 cemented, are sufficiently durable in the mass to withstand all 

 ordinary conditions of exposure. The purer varieties are the best. 

 The presence of argillaceous and siliceous impurities tends to 

 weaken their structure, as there is not the same bond between 

 particles of different nature as exists' between the uniform calcare- 

 ous grains. 



The metamorphic rocks require no special mention. In their 

 mineralogy, they are related to the one or the other of these classes. 

 Metamorphism ordinarily produces small changes in the igneous 

 rocks so far as their mineral ingredients are concerned. With 

 the sediments it tends toward recrystallization of the ingredients, 

 thus making them more compact or harder than the originals, with 

 an approach, in the case of the siliceous sediments, to the struc- 

 tures and mineral contents of the igneous class. 



Texture. There is no doubt that texture (by which is meant 

 the size, form and spacing of the mineral particles) plays an im- 

 portant role in the strength and durability of rocks. The relation- 

 ship, however, is not always so distinct or easily grasped as might 

 be inferred- from the treatment given in some works on quarry 

 materials. As a rule, each quarry presents features that require 

 individual study, not alone by themselves, but with reference to 

 the geological history and mineral content of the material. 



The size of grain obviously affects the appearance and physical 

 qualities of rocks. It is not (contrary to the opinions frequently 

 expressed) an index of their porosity or resistance to weathering 



