HANDBOOK FOR THE DEPARTMENT OF GKOLOGY. 



545 



Geological age and mode of occurrence. — The limestones are metamor- 

 phosed chemical deposits or sedimentary beds occurring associated 

 with rocks of all ages, from the 

 earliest to the most recent. 

 They are considered in large 

 part as rocks composed of the 

 metamorphosed debris of mol- 

 lusks, corals, and other lime- 

 secreting animals. In the pro- 

 cess of metamorphism these 

 remains may have been com- 

 pletely obliterated as in the 

 Vermont marbles (specimen 

 17345), or may still be in part 

 preserved though the entire 

 mass has taken on a crystalline 

 structure, as in numbers 35804, 

 35805, 35907, G9253, etc. To Fig . M . 



the paleontologist this group Micro-structure of Crystalline Limestone;. 



Of rOCks iS One Of Very great (West Rutland, Vermont.) 



importance, owing to their fossil remains. From an economic stand- 

 point they are also of value, furnishing quantities of material for gen- 

 eral building as well as beds of finest marble for ornamental work. 

 (See building and ornamental stone collections.) 



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 marble is applied to any 

 calcareous or magnesian rock sufficiently beautiful to be utilized in 

 decorative work. Argillaceous and siliceous limestones carry clayey 

 matter and sand. Hydraulic limestones contain varying proportions of 

 magnesia, alumina, silica andiron oxides. Such furnish, when burned, 

 a lime with the property of setting under water and known as hydraulic 

 cement. (Specimens 39809 from Roseudale, Ulster County, New York.) 

 Dolomite (so named after the French geologist Dolomieu) is a rock con- 

 sisting of 45.50 per cent, carbonate of magnesia and 54.50 carbonate of 

 lime. (Specimens 37662, 35906, 38820, 36854, 36729, etc.) In its 

 typical form this is distinguishable from limestone by its greater hard- 

 ness and by being less readily soluble in acids. In many cases chemi- 

 cal tests are rendered necessary to distinguish between them, and all 

 gradations are common from limestone with a mere trace of maguesia to 

 those which show the full amount characteristic of dolomite. The in- 

 termediate varieties are spoken of as magnesian or dolomitic lime- 

 stones. (Specimens 37715, 70171, 26559,26209, etc.) The snow-white 

 coarsely crystalline rock from Pleasantville, West Chester County, New 

 York (25046), and Lee, Massachusetts (37662), are good illustrations of 

 dolomites. A chemical analysis of the Pleasantville dolomite as quoted 

 H. Mis. 129, pt. 2 35 



