34 Transactions. 



substances. Thus, while pseudomorphism and metasomatism 

 are closely related processes, it is found that they differ widely 

 in the scope of their operation. Gneiss and mica-schist are 

 familiar examples of the work of segregation and molecular 

 rearrangement of the dominant constituents of sedimentary 

 rocks. 



The internal changes that affect eruptives are known to 

 every petrologist. Besides these changes, which are chiefly 

 molecular, rock-masses, and especially eruptive rocks, may 

 be so altered by the action of circulating waters as to bear no 

 resemblance to the original rock. Thus, in many cases 

 andesites have been changed to propylite by the removal 

 of certain essential constituents and the substitution of others. 



Metasomatic replacement, as defined by Van Hise :|: and 

 Emmons,! does not necessarily imply a mere substitution of 

 matter, molecule for molecule, as happens in the proce-s 

 of pseudomorphism, which involves the preservation of the 

 original form of the substance replaced, but an inter- 

 change of substance, the dissolved rock being replaced by 

 grains or crystalline aggregates of one or more minerals. 

 That substitution did, however, take place in some kinds 

 of deposits is well known. In the tin impregnations found 

 in granite in New South Wales, pseudomorphs of tin in 

 the form of orthoclase are not uncommon ; and many 

 other examples could be quoted having reference principally 

 to the replacement of isolated crystals in crystalline and 

 eruptive rocks. 



Slow replacement of substance by a progressive movement 

 of the solutions in a definite direction must be assumed to 

 have taken place in the formation of ore-deposits composed of 

 massive aggregates of ore and quartzose matrix. J n most cases 

 the direction of movement would be determined by a rock- 

 fracture, fault-line, or crush-zone. In the case of deposits 

 formed by deep-circulating solutions it is manifest that circu- 

 lation could not be rapid, as the face or breast where metaso- 

 matic processes were active would form a blind end or cul-de- 

 sac. Whatever circulation existed would be mainly due to 

 convection currents, which in deep-seated cavities would of 

 necessity be feeble. 



This raises the question as to the transference and supply 

 of dissolved matter to the continually advancing faces of 

 metasomatic action. 



The energy which caused, or, at any rate, accelerated, this 

 transference was probably osmotic pressure, which is a force 



* Van Hise, Sixteenth Annual Report U.S. Geol. Surv., part i, 

 p. 689. 



t S. F. Emmons, U.S. Geol. Surv. Monograph xii, p. 565. 



