ANALYSES OF SLATES. 



895 



The rearrangement of the mineral particles under mass-mechanical 

 conditions is mainly that of reciystallization. The process of granulation, 

 which is so important in connection with reciystallization of the psephites 

 and psammites, is of relatively less consequence in the deep-seated metamor- 

 phism of slate, although by no means unimportant. The particles of the 

 original rocks are so small that they can readily slip over one another with- 

 out being broken finer, so that granulation is mainly restricted to the large 

 particles. The dominance of recrystallization is shown by the fact that 

 the moment the change from shale to slate, schist, or gneiss commenced 

 the small particles begin to merge and a coarser-grained rock is produced. 

 (PL XL) 



The alterations which take place of course come under the general 

 classes so frequently mentioned as characteristic of the zone of anamor- 

 phism — dehydration, silication, and deoxidation. 



In order to get a better idea of the chemical change from mud to 

 shale and then to slate and schist, analyses of slates and schists are here 

 inserted. The average of 12 analyses of clays and soils is given on page 

 891. The average of composite analyses of 78 shales is given on page 890- 

 The following is an average of the analyses of 9 slates from Vermont: 



Average of analyses of 9 slates from Vermont." 



The following is the average of 22 analyses of pelite-slates from 

 various localities in the United States, taken from the publications of 



"Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods, U. S. Geol. 

 Survey, 1880-1896: Bull. U. S. Geol. Survey No. 148, 1897, pp. 277-278. 



