144 NEW YORK STATE MUSEUM 



" soaked " with igneous material and on the other an igneous rock 

 which has meUed into itself or assimilated sedimentary material. 

 Between these two types every gradation exists, thus introducing 

 added complication and uncertainty. 



The pyrite deposits occur in association with the sedimentary 

 rocks, although it is probable that they are more or less closely 

 dependent upon the igneous rocks for their existence. 



Throughout the Grenville series pyrite is a rather common acces- 

 sory mineral and becomes particularly conspicuous in the so-called 

 '' rusty gneisses " which frequently occur. These rusty gneisses 

 owe their name to the dark iron stain resulting from the weathering 

 of pyrite, which is often present in larg-e amounts. The question of 

 the true nature and origin of the rusty gneisses is most intimately 

 connected with that of the pyrite deposits, which in many, if not all, 

 cases are merely exceptionally pyritiferous varieties of these rocks, 

 or of rocks closely associated with them. At the yery outset 

 this makes the problem in hand difficult, since it necessitates the 

 determination of the character of a widespread group of rocks 

 which, though superficially similar, vary in character rather widely 

 and may be of quite diverse origin in different cases. 



In dealing with the Grenville one is on safe ground in classing the 

 crystalline limestones and quartzites as metamorphosed sediments, 

 but with reference to the schists and gneisses the case is quite differ- 

 ent. While many of these may wath confidence be regarded as meta- 

 morphosed shales or impure sandstones and limestones, others are 

 beyond question intrusive sheets and dikes, while it is possible that 

 some may originally have been contemporaneous lava flows and 

 tuffs, though no case of the latter kind has yet been established. 

 Thus while the rusty gneisses appear as part of the Grenville series, 

 it does not necessarily follow that they are of sedimentary origin, 

 though it has become rather habitual to take this view of them. 

 That this course is in general justified seems probable from the fact 

 that these rocks are strongly banded, exceedingly variable in com- 

 position, and so far as studied not closely related in composition to 

 any of the well-established types of igneous rocks. It must be 

 admitted, however, that some examples, marked by the presence of 

 scapolite replacing plagioclase, closely resemble certain varieties of 

 gabbro.^ 



If all of these pyritiferous rocks could be regarded as representing 

 metamorphosed gabbros, a simple solution of the whole problem 



1 Smyth, C. H. jr, Trans. N.Y, Acad. Sci., XII, 1893, p. 215-17. 



