30 NEW YORK STATE MUSEUM 



cumulative in its effects than upon a hill where the products are 

 removed nearly as rapidly as they are formed. In a glaciated 

 country like this State, the presence or absence of bowlder clay is 

 an important feature in determining the effects of weathering. 

 When that material rests directly upon rock, the latter is always 

 much fresher in appearance than when covered with sand or soil. 

 It is now quite generally conceded that no reliable estimate can 

 be made from the weathering qualities of rock in place as to its 

 probable permanency when placed in the walls of a building. That 

 conclusion was reached in the course of an investigation carried out 

 a few years ago by a commission appointed by the Prussian govern- 

 ment. The report of the commission, as quoted from Parks' 

 Building and Ornamental Stone of Canada, 1 stated that: 



i The alterations produced in stone by the agents acting in 

 the crust of the earth are not comparable with those caused by 

 the action of the atmosphere on stone placed in a building. 



2 Changes are produced in the course of the geological ages 

 which can not possibly be effected in the length of time that a 



building stands. 



3 The obtaining of a measure of the time necessary for dis- 

 tinct alteration to appear in a building stone and for the time 

 required for the alteration to proceed through different stages 

 is not assisted at all by observations on geological weathering. 



MICROSCOPIC EXAMINATION 



The microscope beyond all doubt is the most valuable single ad- 

 junct for the laboratory investigation of structural stone. There 

 is no other method that at once yields so many important facts and 

 with so little outlay of time or expenditure for equipment. 



The information which may be had from the examination of 

 rock samples with the microscope include: (i) the identity of the 

 various mineral ingredients, from those of macroscopic size down 

 to the finest particles : sulphides, carbonates and any other harmful 

 components are quickly revealed; (2) the size, form, interlockment 

 or cementation of the grains; (3) the compactness of the rock, or 

 its relative porosity ; (4) the condition of the minerals with respect 

 to weathering; (5) the relative proportion of the different minerals. 

 As minerals are definite chemical compounds, the determination of 

 the relative abundance of each variety affords a measure for reckon- 

 ing the quantitative chemical composition. The results are not so 

 accurate as those obtained by actual chemical analysis, but in ex- 



1 Department of Mines, Ottawa, v. 1, p. 57. 1912. 



