14 NEW YORK STATE MUSEUM 



ages, the character acquired under one set of conditions in spite of 

 exposure to a totally different set of conditions. 



For sedimentary rocks, formed at the surface, with low tem- 

 perature and pressure, the changes effected by metamorphism are 

 distinctly constructive. It is a process of integration and tends to 

 convert relatively soft, incoherent, fine-grained rocks, of fragmentat 

 texture and composed of simple minerals, into hard, dense, coarse- 

 grained, highly crystalline rocks composed of complex minerals. 



Thus, the crystalline limestone that contains the zinc deposits is 

 a coarse, compact marble, carrying crystalline silicates, very dif- 

 ferent from an ordinary unmetamorphosed limestone, drab, blue or 

 black, and containing more or less mud or sand instead of the 

 silicates' crystallized in situ. Even more striking is the contrast 

 between the schists and gneisses, on the one hand, and the muds 

 from which they were derived, on the other. 



METAMORPHISM AND THE ORE DEPOSITS 



As to the kind of metamorphism that has brought about the 

 changes in these rocks, there can be no doubt that both types, so 

 briefly sketched above, have been active in it, though there is room 

 for difference of opinion as to which type predominated. But recent 

 studies in the neighboring Lake Bonaparte quadrangle indicate that 

 limestones of the diopside-tremolite type here represented are largely 

 the product of igneous intrusion or, in other words, the meta- 

 morphism is chiefly of the "contact" rather than the "dynamic" 

 type. But it is contact metamorphism that, on account of heavy 

 cover, widespread intrusion and abundant injection of magmatic 

 material, has operated over such a large area as to be regional, rather 

 than local, in its effects. 



Were the metamorphism purely dynamic, it would follow that the 

 silicates represent only new combinations of the original siliceous 

 impurities with the lime and magnesia of the limestone. But, 

 assuming the metamorphism to be largely contact, it follows that 

 part or all of the silicates may be derived from magmatic sources. 

 The absence of typical pneumatolytic minerals, such as tourmaline 

 and fluorite, would seem to argue against this idea, but experience 

 in other parts of the region suggests that most of the silica and 

 perhaps part of the magnesia have been introduced in hot gases and 

 solutions derived from the igneous magmas that have intruded and 

 so thoroughly injected the sedimentary series. To give the various 

 lines of evidence upon which this conclusion is based, wouM 

 transcend the present limits. 



