436 A TREATISE ON METAMORPHISM. 



The changes in temperature are important in proportion as the rate 

 of change and the amount of change are great. It is rapid change of 

 temperature which mainly results in differential expansion and contraction, 

 and therefore this is the factor of final consequence. The temperature 

 changes are (a) diurnal, (b) cyclonic, and (c) seasonal. The amount 

 of the diurnal change is considerably less than that of the cyclonic, and 

 that of the cyclonic change is considerably less than that of the seasonal ; 

 but the rate of change is far greater for the diurnal than for the cyclonic, 

 and this is far greater than the seasonal rate. Therefore the diurnal 

 changes are the most important in the disintegration of rocks, the cyclonic 

 changes of next importance, and the seasonal changes are of least importance. 



The diurnal changes range from 0° to 50° C. or more. For instance, 

 in Texas the diurnal changes in temperature in the shade are frequently 30° 

 to 40° C." Branner states that in Brazil the diurnal change of temperature 

 from the minimum at night to the maximum in the sun frequently amounts 

 to 50° C, the average appearing to be not far from 40° C. 6 In some 

 desert regions diurnal changes even greater than this have been recorded. 

 Probably even these ranges are not great enough; for the sun beating upon 

 the surface of a rock will raise it to a higher temperature than it will a 

 thermometer in the air. In order to obtain the real effect of the diurnal 

 change of temperature, one ought to sink a thermometer in the exposed 

 rock surface and ascertain its range from the coolest part of the night to 

 the hottest part of the day. 



While the diurnal range is very great, probably the diurnal change of 

 temperature and the resultant fracturing is limited to a few inches in depth. 



The changes in temperature due to the cyclonic period are greater than 

 those due to the diurnal period, but the period is longer and the rate of 

 change is much less. The cyclonic period produces a variation of tempera- 

 ture which runs over several days; its effect majr therefore be felt more 

 deeply than the diurnal range, and it is not improbable that the fractures 

 from a few centimeters to several meters below the surface and jiarallel to 

 the surface in many massive rocks are largely the result of the cyclonic 

 change of temperature. Such fractures are well exhibited in many granite 

 quarries. The scaling probably mainly due to the diurnal and cyclonic 



"Fourth Ann. Rept. Geol. Survey of Texas, 1892, p. 144. 



& Branner, J. C, Decomposition of rocks in Brazil: Bull. Geol. Soc. America, vol. 7, 1896, p. 2S6. 



