Barrett — Physical Effects of Contact Metamorjphism. 289 



morpliism " the percentages of the constituents are given into 

 which the sediments break up in the production of the meta- 

 morphic mineral. 



The latter is the only one of them remaining in the strata, 

 the others escaping as gases, and thus its percentage indicates 

 the amount of shrinkage. 



On being set free the gases expand to many times the 

 original volume of the sediments, the numbers in the table 

 being the volume to which they expand at degrees centi- 

 grade and 760 millimeters pressure, the unit being the original 

 volume of sediments. In computing the volumes of the 

 metamorphosed strata, the porosity factor, being a variable 

 quantity, has not been included, but nevertheless it is seen that 

 in all cases the shrinkage in volume is greater than the shrink- 

 age in weight. 



* Since to use this table it niay sometimes have to be supplemented, the 

 method of computation is given below : 



Grossularite, Ca 3 Al. 2 (Si0 4 )3 



Si0 2 40', A1 2 3 22-7, CaO 37'3 per cent. 

 Silica, Si0 2 , mol. wt. 60' 

 Kaolinite, H 4 Al 2 Si 2 9 , mol. wt. 258 '8. 



2H 2 0=36, Al 2 O 3 =102-8, 2SiO 2 =120. 

 Calcite, CaC0 3 , mol. wt. 100. 



CaO =56, C0 2 =44. 



Using grossularite as a basis of computation. 



^58 "8 

 2-2 '7 x ' — 57'2 ; amt. of kaolinite required by 100 parts of grossularite. 



120 

 57'2 x ^^-5 = 26'5 ; amt. of silica brought in by kaolinite. 



57*2 x — — --= 8'0 ; amt. of water brought in by kaolinite. 



2o8'8 

 40 — 26 - 5=13 # 5 ; amt. of free silica required by 100 parts of grossularite. 



373 x -^- =66*6 ; amt. of calcite required by 100 parts of grossularite. 



44 

 66*6 x =- =29"3 ; amt. of carbon dioxide brought in by calcite. 

 ob 



Thus :— 



57*2 Kaolinite \ ( 100' Grossularite. 



13-5 Quartz J 8*0 Water. 



66-6 Calcite >* y ields < 29*3 Carbon dioxide. 



137*3 Sediments J I 137-3 



Bringing this to a basis in which the amount of sediments shall form the 

 unit gives the following figures : — 



100 parts of sediments consisting of 41-7 kaolinite, 9 - 8 quartz and 48*5 

 calcite yield 72*8 parts of grossularite, 21 "4 carbon dioxide and 5'8 of water, 

 as shown in the table. 



To compute the volumes divide each weight by the specific gravity of the 

 substance, giving the ratio of volumes. Add together those forming the 

 original sediments and compare with the volumes of the several products of 

 metamorphism. The computation for grossularite is as follows : — 



