566 STUDIES FOR STUDENTS 



of the samples that it broke into fragments not much larger than 

 the individual grains. The granites having medium sized grains 

 flaked off at the corners when cooled moderately fast. The fine 

 grained granite, such as the Montello, developed cracks through 

 the middle of the sample. The different ways in which the 

 granites were cracked and broken are illustrated in Plate V. 



In contrast with the limestone and granite, the sandstone 

 was to all appearances little injured by the extreme heat. 

 Samples which were taken from the muffle furnace and allowed 

 to cool gradually appeared to be uninjured, but after they had 

 cooled one could crumble any of them in the hand almost as 

 easily as he could the most incoherent sandstone. In fact, some 

 of the samples when heated to a temperature of 1500 F. became 

 so incoherent that after they had cooled they could scarcely be 

 picked up without falling into sand. A person might be easily 

 deceived as to the injury occasioned by extreme heat on sand- 

 stone. The samples often look as fresh and clean as when first 

 quarried and, unless tested with the hammer, one would never 

 suspect that the strength was so largely gone. 



In general, the temperature tests indicate that there are few 

 if any stones, whether they be granite, limestone, or sandstone, 

 which will effectually withstand for a moderate length of time a 

 temperature of 1500 F. 



MICROSCOPIC TESTS 



Thin sections of the more important building stones which 

 were otherwise inspected were examined under a compound 

 microscope. The microscopic examination reveals clearly 

 the texture, composition and finer structures of the rock. These, 

 combined with the field observations, furnish abundant data from 

 which a person familiar with microscopical studies can estimate 

 both the strength and durability of a stone. 



The irregular interlocking character of the grains composing 

 the igneous and finely crystalline rocks give evidence of strength 

 which far exceeds that displayed by the sandstones composed of 

 rounded grains which are held together by occasional patches of 



