54G REPORT OF NATIONAL MUSEUM, 1890. 



in the circular of the Snowflake Marble Company yielded results as 

 follows : 



Per cent. 



Carbonate of lime 54. 62 



Carbonate of magnesia 45. 04 



Carbonate of iron 0. 16 



Alumina 0. 07 



Silica 0.10 



99. 09 



B. — FOLIATED OR SCHISTOSE. 



1. The gneisses 



Gueiss from the German gneis, a term used by the miners of Saxony 

 to designate the country rock in which occur the ore deposits of the 

 Erzgebirge (Teall). The word is pronounced as though spelled wee not 

 nees. 



Mineral and chemical composition. — The composition of the gneisses is 

 essentially the same as that of the granites from which they differ ouly 

 in structure and origin. They, however, present a greater variety and 

 abundance of accessory minerals, chief among which may be mentioned 

 (besides those of the mica, hornblende or pyroxene group) garnet, tour- 

 maline, beryl, sphene, apatite, zircon, cordierite, pyrite, and graphite. 



Structure. — Structurally the gneisses are holo-crystalline granular 

 rocks as are the granites, but differ in that the various constituents are 

 arranged in approximately parallel bands or layers as in specimens 

 72862 aud 72SG3, from Madison County, Montana, and 36180 from Bra- 

 zil. In width and texture these bands vary indefinitely. It is common 

 to find bauds of coarsely crystalline quartz several inches in width, 

 alternating with others of feldspar, or feldspar, quartz and mica or 

 hornblende (see specimen 72SC2). A lenticular structure is common, 

 produced by lens-shaped aggregates of quartz or feldspar, about and 

 around which are bent the hornblendes or mica laminae, as in specimen 

 20517 from Massachusetts, in the structural series. The rocks vary 

 from finely aud evenly fissile through all grades of coarseness and be- 

 come at times so massive as to be indistinguishable from granites in the 

 hand specimens. The causes of the foliated structure are mentioned 

 below. 



Colors. — Like the granites they are all shades of gray, greenish, pink 

 or red. 



Geological age and mode of occurrence. — The true gneisses are among 

 the oldest crystalline rocks, aud are considered by many geologists as 

 representing " portions of the primeval crust of the globe, traces of the 

 surface that first congealed upon the molten nucleus." By others they are 

 regarded as metamorphosed sedimentary deposits resulting from break- 

 ing down of still older rocks, and may not in themselves therefore be 

 confined to any one geological horizon. (See specimen conglomerate 

 gueiss from Maine, No. 30018). They are in large part, however, iudis- 



