1200 A TREATISE ON METAMORPHISM. 



VARIATIONS IN POROSITY AND - STRUCTURE. 



It has been fully explained that a vigorous circulation is essential for 

 the production of aqueous ore deposits. It lias been pointed out that in 

 order to produce an ore deposit the amount of water which must flow 

 through the openiugs is many thousands of times the volume of the ores 

 deposited. Also it has been explained that waters do not produce channels. 

 In order to have a vigorous circulation original and continuous channels 

 must exist. Thus initial porosity in rocks is an essential factor in the devel- 

 opment of ore deposits. In the foregoing discussion the existence of trunk 

 channels has been recognized, but otherwise it has been assumed that 

 the rocks were equally porous in all directions. As a matter of fact, the 

 rocks range between the widest extremes in porosity; from those in which 

 the openings are subcapillary and which are substantially impervious to 

 those which are as open as a sieve in all directions. 



The different masses of sedimentary rocks, igneous rocks (especially 

 the lavas), and metamorphosed equivalents of either sedimentary or igneous 

 rocks may differ in porosity." The contacts of rocks of different kinds fre- 

 quently furnish trunk channels for circulating water. Bedding partings 

 produced by shearing stresses during deformation furnish sheet channels 

 parallel to the strata, or openings on the anticlines or synclines. Some 

 strata when deformed vna,y yield by fracture, furnishing channels for water 

 circulation, while interlaniinated strata may yield by flowage, thus remain- 

 ing relatively impervious. These various irregularities may combine in 

 different ways. 



All irregularities in porosity and structure require modification of the 

 simple general statements of the present paper (see pp. 129-156, 571-589) 

 concerning the character of underground circulation and the concentration 

 of ore deposits. At some time it may be possible to divide the modifica- 

 tions of the general circulation due to variation of porosity and structure 

 into classes, but at present this can not be done. The modifications of the 

 general circulation which occur in many districts must first be studied and 

 described individually, after which generalizations may possibly be made. 

 At present some statements may be made in reference to certain modifica- 

 tions of the underground circulation. 



« Compare Emmons, Structural relations of ore deposits: Trans. Am. Inst. Min. Eng., vol. 16, 

 18SS, pp. 80J-839. 



