434 



A TREATISE ON METAMORPHISM. 



the number at the limit of vision was always great."" It thus appears that 

 in the case of chemical subdivision it is doubtful if the microscope used 

 fully resolved the material into its constituent particles. 



CHANGE IN TEMPERATURE. 



Change in temperature results in the disin- 

 tegration of the rocks by producing differential 

 expansion and contraction. When the rock 

 surface is affected by any change in tempera- 

 ture, the heat is only slowly conducted and there 

 is differential expansion between the outer part 

 of the rock and the inner part. In other words, 

 powerful shearing stresses are set up, as a result 

 of which flakes of the rock are disrupted from 

 the deeper located solid material (see fig. 6). 



The surface temperature may be greater or 

 less than the average temperature of the rock, 

 and the proce;: :c especially effective when the 

 surface temperature is •alternately lower and 

 higher than the average temperature. 



The expansion of different rocks per degree 

 fig. 6.-ESect oi unequal heating of the ce utigrade varies from less than 0.000023 cm. to 



surface of a rock, (a) shows the con- 

 dition of a block of uniform temper- about 0.000046 cm. per meter. When these 



attire. (6) illustrates the manner in _ 



which the upper portion of a rock numbers are multiplied by the number ot degrees 



surface expands when heated above • . . , „ 



average temperature; where the differ- OI change dul'Ulg ally pei'lOCl as, IOl* lUStailCe, 



ence in> temperature is sufficiently -, , i , -i r i 



great, this results in the splitting off one day-<-we have the increment ot change in 



llustratesthe ^ j ^ q£ ^ ^^ ^ ^ gurface fc^ the 



contraction of the upper surface by 

 cooling below the average tempera- 

 ture; where the difference in temper- 

 ature is sufficiently great, this results 

 in the splitting off of the upper layers. 



period. Suppose the change for a given day to 

 be 40° C, and the length of the rock surface be 

 taken as 1 meter at the mean temperature, then 

 its length at the coolest period will be from 0.99908 to 0.99954 cm., and at 

 the hottest period from 1.00046 to 1.00092 cm. A few inches below the 

 surface it may be assumed that the temperature of the rock is near the 

 mean; consequently, at the coolest part of the day the surface of the rock 

 is shorter than the rock below, and at the hottest part of the day it is 



« Charnberlin and Salisbury, cit, p. 248. 



