HANDBOOK FOR THE DEPARTMENT OF GEOLOGY 



547 



putably the oldest known rocks, lying beneath or being cut by all rocks 

 of later formation or injection. 



The origin of the gneisses as above suggested is in many cases some- 

 what obscure, the banded or foliated structure being considered by 

 some as representing the original bedding of the sediments, the differ- 

 ent bauds representing layers of varying composition. In many schis- 

 tose rocks this structure is now, however, considered to be due to 

 mechanical uses, and in no way dependent upon original stratification 

 (see Geikie's Textbook, pp. 123 and 298). The name as commonly used 

 is made to include rocks of 

 widely different structure, 

 and which are beyond doubt 

 in part sedimentary and in 

 parteruptive,butinall cases 

 altered from their original 

 conditions. Figs, land 2 on 

 PI. cxxiv show two rather 

 extreme types of these 

 rocks. Fig. 1 is that of a- 

 banded gneiss from Madi- 

 son County, Montana (speci- 

 men 72862), and which, so 

 far as we know, may be an 

 altered sedimentary rock. 

 In fig. 2 of the same plate 

 (made from specimen No. 

 26547 in the building stone 

 collection) is shown a folia- 

 ted rather than a banded 

 rock, and whatever may have been its origin it undoubtedly owes its 

 foliated structure to dynamic agencies. The effect of the shearing 

 force whereby the foliation was produced is evident in the figure, even 

 to the unaided eye, to the left and just above the center, where an elon- 

 gated feldspar is seen broken transversely into four pieces. The same 

 features are shown even more plainly in fig. 94, which shows the struc- 

 ture of this same gneiss as seen under the microscope. 



As in the present state of our kuowledge it is in most cases impossi- 

 ble to separate what may be true metamorphosed sedimentary rocks 

 from those in which the foliated or banded structure is in no way con- 

 nected with bedding and which may or may not be altered eruptives, 

 all are grouped together here. 



Classification and nomenclature. — The varietal distinctions are based 

 upon the character of the prevailing accessory mineral as in the granites, 

 forming a parallel series. We thus have biotitegneiss, muscovite-gneiss, 

 biotite-muscovite-gneiss, hornblende-gneiss, etc. Rarely the mineral cor- 

 dierite occurs in sufficient abundance to become a characterizing acces- 

 sory, as in specimens 73097 and 73178 from Saxony and Connecticut. 



Fig. 94. 



MlCROSTKUCTURE OF GNEISB. 



(West Andover, Massachusetts.) 



>n No. 26.MI5. At a a lire shown plngiocla 



upressr 



:fon 



le crystals broken 

 foliation.) 



