222 THE CEYSTxVL FALLS IROF-BEARING DISTRICT. 



granite (plagioclastic), and, on the other, hornblende gabbros, gabljros, 

 norites, and, lasth', peridotites of varying niineralogical character. These 

 rocks thus resemble in their variations those Scottish plutonic rocks so well 

 described by Messrs. Dakyns and Teall.^ 



The rapid changes in mineralogical composition and texture in a single 

 rock, and the changes from one kind to another through intermediate facies, 

 show very clearly the intimate relationship of these rocks to one another, 

 and warrant the assumption that they all belong- to a geologic unit, a con- 

 clusion readied a number of years since by Williams for a somewhat sim- 

 ilar series, the Cortlandt series, from New York. 



Granite is present as a local facies of the diorite. However, it is very 

 subordinate in quantity and not altogether typical, and as no analysis has 

 been obtained, its position is still more or less doubtful. 



In the following pages only those kinds of rocks of which analyses 

 have been obtained will be included in the final discussion. Others will be 

 described in detail or merely mentioned, as representing facies of the main 

 types, according to their petrological interest. The rock tj-pes of which 

 analyses have been made are as follows: Diorite, gabbro, uorite, and 

 peridotite. 



DIORITE. 



NOMENCLATURE. 



Diorite, according to the generally accepted definition, is a granular 

 rock consisting essentially of hornblende, which must be j^rimary, and a 

 soda-lime feldspar.^ The term has been used in a different sense by many 

 writers on the Lake Superior and other regions. It has. been used to com- 

 prise rocks Avhich contain hornblende and plagioclase as preponderating 

 constituents, it is true, but in which the hornblende is secondary, therein 

 differing from a true diorite. These so-called diorites have been regarded 

 as deri-s-ed from an original dolerite (diabase) b}^ uralitization of the pyrox- 

 ene. By some writers these rocks have been classed Avith the epidiorites, 

 thus recognizing their secondary nature, but by this name, epidiorite, 

 unfortunately implying a false relationship. 



In this paper, following Brogger, I restrict the name to the granitic 



' Oil tUe iilutonic rocks of Garabal Hill and Meall Broac, by J. I{. Dakyus, es(i., M. A., and J. J. 

 H. Teall, esq., M. A., F. R. S., F. G. S. : Quart. .Jour. Geol. Soc, Vol. XLVIII, 1892, pp- 101-121. 

 -Lebrbucli der Petrograj)liic, by F. Zirkel, Leipzig, Vol. II, 1894, p. 465. 



