288 NATURE AND ORIGIN OF SERPENTINES. 



Dunite is decomposed into serpentine. A similar rock appears at 

 Kranbat in Upper Styria, and at several places in the Vosges, near 

 St. Etienne, &c. There is rarely any enstatite or diallage, but both 

 are found, and are sometimes decomposed into chlorite or amphibole. 



Serpentine. 



Serpentine is now known to result from reconstruction of the 

 mineral constituents of many kinds of igneous rocks, especially of 

 those rich in olivine and enstatite. The whole group of peridotites or 

 olivine rocks may be converted into serpentines, though a few serpen- 

 tines have been described to which no such origin can be attributed. 

 Examples of divine-serpentines have been enumerated under the 

 peridotites ; but at Todtmoos in the South Schwarzwald, a serpentine 

 is seen which indicates that the rock from which it was formed con- 

 sisted of a mixture of diallage and enstatite, and contained as acces- 

 sories biotite and hornblende. In the Bluttenthal in the Vosges, the 

 serpentines are said to be decomposed hornblende slates. But even 

 in these cases it is difficult to affirm positively that the rock 

 was always- free from olivine ; though nothing is needed for the 

 production of serpentine but an abundant infiltration of silicate of 

 magnesia in chemical combination with water. The ground mass is 

 colloidal, and contains crystals and fibres of the minerals which it 

 replaces. It is typically green. When serpentine occurs as an 

 accessory, it may replace olivine, hypersthene, augite, or amphibole ; 

 but black mica, amphibole, and augite are more frequently converted 

 into chlorite. MM. Fouque and Levy regard the viridite of volcanic 

 rocks as a phase of serpentine. Any rock which includes these 

 minerals may be more or less converted into serpentine. When 

 serpentine results from the decomposition of enstatite, bastite is fre- 

 quently formed. 1 Serpentine often occurs in schists, when it may be 

 due to the presence of glauconite in the original sedimentary rock 

 which was metamorphosed in other cases it is certainly intrusive, but 

 even then it may have had a similar metamorphic origin. 



Serpentine in Relation to Diallage Rock. The abundance of 

 serpentine in the Pyrenees, Apennines, and other parts of the south 

 of Europe, has long been known. Diallage rocks, which are equally 

 abundant, often occur in connection with the serpentine, and there is 

 now no doubt as to the fact that these two rocks are very intimately 

 related. Few conclusions of this nature appear better authenticated 

 by observation than the gradation of diallage rock into serpentine, in 

 the Alps, the Apennines, Corsica, and Cornwall. 



In the Northern Apennines Brongniart remarked the following 

 general order of succession downwards : i. Serpentine. 2. Diallage 

 rock, in the upper part assuming the aspect of serpentine (at Eochetta, 

 north of Borghetto, near Spezia), consisting partly of red crystallised 

 limestone. 3. Jasper rock in thin laminae. Below these are lime- 

 stones and marly schists, common in the Apennines. In Monte 

 1 Fouque* and Levy, Roches Eruptives Fransaises, 1879. 



