62 P)'of. J. D. Dana — Metamorphism of 



The former include four groups, liased on the iron-bearing silicate 

 prominent in the kinds ; (A) the Hornbhndic; (B) the Ilypersthemc ; 

 (C) the Angiiic ; and (D) the Micaceous or Biotitic ; but the groups 

 pass into one another by intermediate varieties. The chrysolite- 

 bearing kinds are either (E) hornblendic, or (F) augitic, or (G) 

 chiefly chrysolite; but here again intermediate kinds occur. 



In the following descriptions I have confined myself to noting 

 only the prominent distinctions so far as necessary to the strati- 

 graphical discussion beyond.' 



A. The Sornhlendic. — The common hornblendic rock resembles 

 syenite, but contains little orthoclase and much triclinic felspar. 

 The latter is mostly of the species oligoclase, according to an optical 

 measurement on cleavage slices. In addition, quartz is rather 

 sparingly present. The rock contains more or less black mica and 

 sometimes much of it ; and as the mica increases at the expense of 

 the hornblende, the rock passes into soda-granite (mentioned beyond). 

 The quartz-diorite has the same relation to soda-granite that quartz 

 syenite has to potash or common granite. Garnets are rare. The 

 rock is often a very coarsely crystallized rock (.4a), having the horn- 

 blende crystals large, one-fourth to one-half an inch being a common 

 size, and an inch and larger also common; and not unfrequently the 

 black cr^'stals are as large as the fingers, and occasionally six to 

 eight inches long. A fine-grained variety {Ah) has a blackish 

 colour; and this variety is sometimes porphyritic {Ac). The 

 micaceous is another common variety {Ad). Another kind {Ae) is 

 exceedingly fine-grained, and consists of minute grains of horn- 

 blende along with similar felspar grains, which are partly ortho- 

 clase ; it looks much like hornblende schist, and in some places is 

 schistose. It sometimes contains an occasional crystal of hyper- 

 sthene. Another rock of the region is hornblendite, consisting almost 

 wholly of hornblende. One variety (-4/) is made up of coarsely 

 crystallized black hornblende ; another {Ag) of grey hornblende 

 along with an asbestiform mineral. The black hornblendite 

 graduates into diorite : and again, it is often chrysolitic. 



B. The Hypersthenic. — The typical rock of this division {Ba) is 

 the most widespread of the Cortland series. It consists of triclinic 

 felspars (and, according to some trials, yet incomplete, by Dr. 

 Hawes, oligoclase is the most abundant), hypersthene in grains or 

 quite small crystals, with frequently more or less biotite, and often 

 some orthoclase. There are also present some magnetite and apatite, 

 frequently traces of quartz, and generally some augite or hornblende. 

 In mineral constitution it approaches one of the kinds of rock that 

 have been called both norite and hypersthenite or hyperite. The 

 name norite is here given it pi'ovisionally. It, however, looks more 

 like a coarsish dolerite or diabase than like other hyperstliene rocks. 



The norite has commonly a dingy brownish-red colour on a 

 surface of fracture, owing to the smoky-red colour of the felspar, 

 but varies from this to pale grey on one side and blackish-grey 



' A detailed study of the rocks of the Cortland region has already been begun, at 

 my suggestion, by the accomplished lithologist, Dr. G. W. Hawes. 



