374 REPORT ON NATIONAL MUSEUM, 188G 



fossiliferous ; in short, may show all the variations common to ordinary 

 limestones, from which they can usually be distinguished only by chem- 

 ical tests. Many marbles are magnesian, as will be noticed by reference 

 to the tables. When the carbonate of magnesia in a limestone rises as 

 high as 45. G5 per cent, the rock is no longer called magnesian lime- 

 stone, but — 



DOLOMITE. * 



This in its typical form is a crystalline granular aggregate of the 

 mineral dolomite, and is usually whitish or yellowish in color. It can 

 in its typical form be distinguished from limestone by its increased 

 hardness (3.5-4.5) and specific gravity (2.8-2.95). It is also less sol- 

 uble, being scarcely at all acted on by dilute hydrochloric acid. Dol- 

 omite shows all the peculiarities pertaining to limestones, both in color 

 and texture, and a chemical analysis is often required to distinguish 

 between them. The pure white marble from Cockeysville, Md., is a 

 dolomite, but by the eye alone can not be distinguished from the white 

 crystalline limestones (marbles) of Vermont. The red-mottled marbles 

 of Malletts Bay, Vt., are also dolomites, as are the white marbles of 

 Lee, Mass., and Pleasantville, N. Y. 



In composition there is no essential difference between a limestone or 

 dolomite and what is popularly called a marble, but for convenience sake 

 the subject will be here treated in two parts, the first to include those 

 of this class of rocks as are put upon the market as marbles, and the 

 second the rocks of the same composition but unfit for finer grades of 

 building and ornamental work and known popularly as simply lime- 

 stones. 



(3) LIMESTONES AND DOLOMITES. MARBLES. 



Under the head of marbles then are here included all those rocks con- 

 sisting essentially of carbonate of lime (limestone) or carbonate of lime 

 and magnesia (magnesian limestone and dolomite) that are susceptible 

 of receiving a good polish and are suitable for ornamental work. 



Alabama. — Beds of marble of great beauty are stated to occur along 

 the Cahawba Biver in Shelby County of this State. The colors enumer- 

 ated are gray with red veins, red and yellow, buff with fossils, white 

 crystalline, clouded with red and black. A black variety veined with 

 white occurs on the road from Pralls Ferry to Montevallo and on Six 

 Mile Creek. Other good beds are stated to occur on the Huntsville 

 road about 19 miles from Tuscaloosa and at Jonesborough, the latter 

 rock being compact and of a red and white color ; the same strata oc- 

 curs at Village Springs. On Big Sandy Creek good marbles occur simi- 

 lar to those on the Cahawba. t None of the above are actively quarried, 

 and the writer has had the opportunity of examining but a single 



* So called after the French geologist, Dolomieu. 

 tGeol. of Alabama, First Bien. Rep., 1849, p. 45. 



