16 Diorites and Granites of Swifts Creek, 



diorites and granites, contributed to these deposits.* Some 

 Upper paloeozoic conglomerates which have been derived 

 from those older formations also contributed, and other 

 sources may have existed in the abraded and worn rock 

 masses, which we cannot now point out. 



Definitions of the Terms Used in this Essay to 

 Describe the Igneous Rocks. 



Felsite is a compact or micro-crystalline granular com- 

 pound of orthoclase-felspar and quartz. When it contains 

 orthoclase crystals it is porphyritic felsite (felspar porphyry) ; 

 when it contains quartz crystals it is a quartz felsite (quartz 

 porphyry) ; when the felspar is not orthoclase, but plagio- 

 clase, the rock is a plagioclase felsite. 



Granitite is composed of quartz, orthoclase, plagioclase, 

 and magnesia mica."f 



Amphibole granite is composed of quartz, orthoclase, 

 plagioclase, and hornblende.*}* 



Diorite is composed of plagioclase and hornblende ; plagio- 

 clase and magnesia mica constitute mica-diorite. The 

 addition of quartz to either of the above forms a quartz- 

 diorite or a quartz mica diorite. 



Amphibolite is a rock composed essentially of more or 

 less perfect crystals of amphibole confusedly aggregated 

 together. 



Diabase is a granular compound essentially of plagioclase 

 and augite, to which are associated chlorite and magnetite. 

 The term is applied to the pretertiary representatives of 

 dolerite. 



To the above is added the contact rock aplite or semi- 

 granite, which is a granular compound of potash felspar 

 (orthoclase or microcline) and quartz, with muscovite mica 

 as an accessory, and, in very rare cases, plagioclase. 



Note. — The crystallo-graphic notation made use of is that of Professor 

 J. D. Dana. 



* In the New London Reef, Happy Valley, near Myrtleford, I found that 

 the auriferous rock worked was, in fact, a very quartzose variety of hornfels. 

 The auriferous portion could not be called a true vein, but resembled more 

 some of the contemporaneous segregations of quartz found in the 

 crystalline schists. 



f Rosenbusch Mihroshopische Physiographie der Massigen Gesteine, 

 p* 19. 



