DOLOMITE. — SERPENTINE. 



77 



Dolomite, so called in honor of the French geologist Dolomieu, is 

 a variety or modification of limestone ; it contains 48 parts of mag- 

 nesian earth, combined with 52 parts of calcareous earth. Dolomite 

 is found in rocks of different classes ; that which occurs at St. Go- 

 thard, and other parts of the Alps, closely resembles white primary 

 limestone : it is minutely granular, and the grains are easily separa- 

 ted by the finger ; but some varieties are harder. Dolomite and the 

 magnesian limestones in the secondary strata, dissolve with more 

 difficulty in acids than common limestone. Dolomite forms vast 

 beds in the western Alps ; it occurs also in various parts of the Ap- 

 pennines; in Carinthia there are entire mountains of Dolomite. The 

 beds of Alpine Dolomite are often much broken, apparently by the 

 protrusion of beds and masses of porphyry. The eminent geologist 

 von Buch maintains, that limestone has been converted into Dolo- 

 mite by its proximity to porphyry in fusion, and that the magnesia 

 has been transferred from magnesian minerals in the porphyry to the 

 limestone ; the magnesia being reduced to vapor or gas. Great diffi- 

 culties attend this theory ; I shall hereafter notice situations in Eng- 

 land, where the theory might be subjected to the test of direct ex- 

 periment. For the present, it may be sufficient to notice, that many 

 strata of magnesian limestone appear far removed from the possible 

 influence of igneous rocks. Magnesia is found in many earthy min- 

 erals, and may be regarded as a constituent element of the globe. 



Serpentine derives its name from its variegated colors and spots, 

 supposed to resemble the serpent's skin ; its chemical composition 

 has been before described. The colors are, most generally, various 

 shades of light and dark green, which are intermixed in spots and 

 clouds; some varieties are red. When fresh broken, it has some 

 degree of lustre, and a slightly unctuous feel ; when pounded, the 

 powder feels soapy. It is harder than limestone, but yields to the 

 point of a knife, and will receive a very high polish. When serpen- 

 tine is found intermixed with patches of crystalline white marble, it 

 constitutes a stone, denominated verde-antique, which is highly val- 

 ued for ornamental sculpture. Some varieties of serpentine are trans- 

 lucent, in others, there is an appearance of crystallization, forming a 

 mineral called diallage or schiller spar. The minerals associated with 

 serpentine are generally those allied to talc. Compound rocks in 

 which talc and hornblende are predominating ingredients, pass into 

 serpentine. Magnesia enters, largely, into the composition of these 

 rocks. A late analysis of one kind of serpentine, gave forty eight 

 per cent, of this earth. Serpentine, commonly occurs in gneiss and 

 mica-slate, in beds, which are sometimes so thick as to compose 

 mountain masses of considerable height. Serpentine, sometimes, 

 becomes magnetic, from an intermixture whh minute particles of 

 magnetic ironstone. Many of the alpine districts in Europe contain 

 rocks and beds of serpentine ; but, according to Patrin, there is no 

 serpentine in northern Asia, nor was it seen by Humboldt in the Ah- 



