Varieties of Gneiss. 



383 



ed as varieties of one another : differing only in the mode of their 

 production, as in the case of the stratified and unstratified limestones 

 and serpentines. Little advantage, however, would be gained by such 

 an innovation ; and granite and gneiss have so long been considered 

 as classical terms in geology, that if possible they ought to be retain- 

 ed ; lest that neological spirit, which vain ambition nourishes, should 

 unsettle every principle of the science. 



The gneiss of Massachusetts corresponds almost exactly to that de- 

 scribed by European geologists, particularly by Dr. Macculloch. 

 Our gneiss, however, does not to my knowledge alternate with clay 

 slate, as it does in Scotland. I am inclined also to believe, that ours 

 exhibits a greater regularity of stratification, producing a fine rock 

 for architectural purposes. 



Although in general the characters of gneiss are tolerably distinct, 

 yet an almost infinite variety of specimens may be obtained, slightly 

 differing in the color, arrangement, or proportion, of the ingredients. 

 They may, however, be reduced to a few leading varieties. 



Miner alogical Characters. 



1 . Granitic Gneiss. ( Nos.972 to 992. ) I suppose this variety to be the 

 granite-gneiss of Humboldt. It certainly approaches very near to 

 granite; and in hand specimens cannot be distinguished from a coarse 

 grained variety of that rock. Even for an extent of several yards, we 

 can sometimes discover no marks of a schistose or stratified structure ; 

 but those structures, at least the schistose, usually appear at no great 

 distance, to prevent our regarding the rock as granite. It might, in- 

 deed, on a superficial view, be considered as a vein of granite in gneiss. 

 But the masses will be found too irregular for such a supposition ; 

 and often they are bounded on all sides by well characterized gneiss. 

 It would explain the mode in which this rock presents itself, to suppose 

 that a slaty rock was once in a state of partial fusion, while some por- 

 tions of it were perfectly melted, so as to crystallize in the form of a 

 coarse granite ; and the principal part of the mass cooling before the 

 slaty structure was entirely lost. In traveling miles I have some- 

 times been at a loss to decide whether the rock were gneiss or gran- 

 ite, until a very careful examination disclosed a partially obsolete 

 parallelism of the mica. I think, however, that there is a slight pe- 

 culiarity of aspect in most of the granitic-gneiss of Massachusetts, 

 which would enable me to distinguish it from genuine granite even 

 in hand specimens, which were wholly destitute of a parallel disposi- 



