382 Ball — P re-Canibrian Rocks of Georgetown, Col. 



apparently being absorbed in the production of the hornblende. 

 Large poikilitic plates of biotite have already been mentioned 

 as characterizing some outcrops and small biotite blades are 

 rather constant constituents. Biotite is also secondary to horn- 

 blende. The accessory minerals include magrotite, ilmenite, 

 pyrite, and large and abundant crystals of apatite. Zircon 

 and rutile are less common. Apatite crystals enclosed in 

 feldspar have embayed borders as if magmatically corroded. 



Diorite upon the borders of the intrusive masses is some- 

 times mashed to a gneissoid rock partly through reerystalliza- 

 tion and partly through granulation. Hornblende and feldspar 

 are segregated into lenses rudely elongated parallel to the part- 

 ing, and blades of biotite and a colorless amphibole, probably 

 an thophyllite, are developed parallel to the gneissic plane. The 

 albite twins of the plagioclase are sometimes aligned parallel 

 to the gneissic structure, while the undulose extinction of 

 orthoclase passes into the " gitter " structure of microcline, 

 the latter mineral being clearly secondary to the former and 

 occurring solely in mashed facies of the diorite. 



Associated with the diorite and linked to it by some grada- 

 tional facies are fine-grained, granular rocks of greenish black 

 color which are perhaps best styled hornblendites. Under the 

 microscope some of these, with the exception of minor quanti- 

 ties of plagioclase, quartz and accessory minerals, are formed of 

 green hornblende ; others are made up of large irregular green 

 hornblende individuals enclosing poikilitically laths of biotite, 

 grains of enstatite showing schillerization and partial columns 

 of a colorless amphibole, probably anthophyllite. In some 

 slides this amphibole, which alters to talc, is as abundant as 

 hornblende. Still another of these rocks is in one portion of 

 the slide composed entirely of hornblende and in another 

 largely of white monoclinic pyroxene near malacolite. These 

 fine-grained rocks have suffered considerable recrystallization 

 and their original character is in doubt. 



Age. — The quartz-bearing diorite bears the same structural 

 relations to the other formations of the Georgetown quadrangle 

 as does the quartz-monzonite. The contact of the two forma- 

 tions is nowhere well exposed in the quadrangle, although inter- 

 mediate types indicate that the two may be variants of the same 

 magma. Near St. Mary's Lake, north of the Georgetown quad- 

 rangle, the two appear at some places to grade into one another 

 and in others the diorite clearly cuts the quartz-monzonite. 

 The two are believed to be differentiation products of the same 

 magma, the quartz-bearing diorite on the whole being slightly 

 younger. 



