632 



A TREATISE ON METAMORPHISM. 



rocks, especially the lavas, contain more pore space than 10 or 12 per cent. 

 This is especially likely to be true of the basic igneous rocks, of which the 

 basalts may be taken as illustrative. But in basalt there is ordinarily no 

 quartz. Practically the entire mass of the rock is composed of silicates, and 

 the complete alteration of these compounds would furnish a sufficient amount 

 of material, supposing the oxidation, carbonation, and hydration to produce 

 an expansion of volume of 25 per cent, and all the compounds formed remain 

 in situ, to fill a pore space of one-fifth the volume of the rock. 



As an illustration .of the very considerable amount of enlargement in 

 volume by metasomatism in the belt of cementation a specific case may be 

 given. For this purpose we may take a rock of intermediate composition. 

 The amphibole-gabbro of Beaver Creek, Big' Trees quadrangle, California, 

 the mineralogical composition of which has been determined by Cross, 

 Iddings, Pirsson, and Washington, will serve the purpose. 01 Columns (1) 

 to (3) are taken from these authors: 



Table showing relative volumes of original and altered rocks in belt of cementation. 



a) 



Mineral. 



Orthoclase 



Albite 



Anorthite 



Diopside 



Hypersthene . . 



Olivine 



Magnetite 



Ilmenite 



Apatite 



Pyrite 



Undetermined . 



Total. 



(2) 

 Formula. 



K,Al,Si 6 16 . 

 Na 2 Al 2 Si 6 16 . 

 CaA.l 2 Si,0 8 . . 

 CaMgFeSisOi, 

 MgFeSi 2 O .- 

 Mg 2 Fe 2 SiA- 



Fe 3 0, 



FeTiO s 



CajPA 



FeS, 



(3) 

 Per cent. 



1.11 



23.06 



43.92 



13.10 



3.92 



7.75 



2.55 



1.67 



1.55 



.20 



1.37 



W 



Volume 



ratio. 



100. 20 



0. 8743 

 1. 2082 



1. 3465 

 1. 2788 

 1.1284 

 1. 1519 

 1. 0000 

 1. 0000 

 1. 0000 

 1. 0000 

 1. 0000 



(S) 

 Volume of 

 resultant 

 products. 



0. 9705 

 27. 8611 

 59. 1383 

 16. 7523 



4. 4233 



8. 9272 



2.55 



1.67 



1.55 

 .20 



1.37 



125. 4127 



Supposing that the orthoclase by hydration and carbonation passes into 

 kaolin and quartz, the decrease in volume is 12.57 per cent (p. 389). Sup- 

 posing that the albite passes into analcite and quartz by simple hydration, 



"Cross, Whitman, Iddings, J. P., Pirsson, L. V., and Washington, H. S., Quantitative classifica- 

 tion of igneous rocks, Univ. of Chicago press, 1903, p. 199. 



