142 



METAMOEPHIC ROCKS 



Geological Age and Mode of Occurrence. — The crystalline 



limestone and dolomites 

 are but the metamor- 

 phosed sedimentary de- 

 posits such as have al- 

 ready been described on 

 p. 125. They occur asso- 

 ciated with rocks of all 

 ages, but only in regions 

 that have been subjected 

 to disturbances such as 

 the folding and faulting 

 incident to mountain- 

 making, or the heat from 

 intruded igneous rocks. 

 From an economic stand- 

 point, the rocks of this 

 group are of great eco- 



Fig. 14.- 



-Microstructure of crystalline lime- 

 stone (marble). 



nomic value for structural and decorative purposes. 



Classification and Nomenclature. — It is common to speak of 

 this entire group of rocks as simply limestones, though many 

 varietal names are often rather indefinitely applied. The name 

 marUe is given to any calcareous or magnesian rocks of snch 

 quality as to be utilized in decorative work or high grade con- 

 struction. Argillaceous and siliceous limestones carry clayey 

 matter and sand. Dolomite (so named after the French geologist 

 Dolomieu) consists of 45.50% carbonate of magnesia and 54.40% 

 carbonate of lime, as already noted. The names ophioUte and 

 ophicalcite are popularly applied to stones consisting of a granu- 

 lar aggregate of calcite and serpentine, such as occur in Essex 

 County, New York, and are used as marbles. 



2. FOLIATED OR SCHISTOSE 



(1) THE GNEISSES 



Gneiss, 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. The word is pronounced as 

 though spelled nice. 



Mineral and Chemical Composition. — The composition of the 



