656 A TREATISE ON METAMORPHISM. 



In the porous rocks, such as sandstones, extrusive igneous rocks, 

 especially the vesicular lavas and porous tuffs, and much fractured rocks, 

 where there is comparatively rapid circulation of water, the migration of 

 materials for considerable distances may be important. The addition or 

 subtraction of material at any place may be large; for instance, simply 

 filling the pore spaces may require the addition of 20 to 40 per cent of the 

 volume of the rocks! (See pp. 569-570.) Also alterations extend through 

 the minerals originally present. Therefore the chemical compositions of the 

 rocks for extensive areas may be much changed. Excellent illustrations of 

 the transportation of material for long distances and its replacement of other 

 material, thus changing the composition of the rocks, are furnished by the 

 segregation of individual minerals, described on. pages 645-646. One of the 

 best illustrations of extensive change in chemical composition is furnished 

 by the silicification of limestone formations. (See pp. 646, 816-820.) 

 Perhaps there is no more conclusive case of replacement than the very 

 numerous instances of partial and complete substitution of copper for 

 porphyry pebbles and bowlders, described by PumpeiTy," in the Calumet 

 and Hecla conglomerate. Other illustrations of replacement are furnished 

 by many of the ore deposits. 



«Pumpelly, Raphael, The paragenesis and derivation of copper and its associates on Lake Superior: 

 Am. Jour. Sci. for Sept., Oct., and Nov., 1871, vol. 2, pp. 188-198, 243-258, 347-355. 



