424 TALCOID SCHISTS. 



sublimation of carbonate of magnesia, do not all satisfy the facts as we now understand 

 them. The views of Bischof are given in the following language: " Taking into considera- 

 tion all the facts known with regard to dolomite, so far as it occurs as a rock mass, it can 

 only be regarded as a product of the alteration of limestone in the wet way ; and there is 

 no mode of alteration that is more probable than the substitution of carbonate of magnesia 

 present in water for a portion of the carbonate of lime in limestone, or the extraction of 

 the greater part of the carbonate of lime by the water permeating the limestone." 



We are not sure that the views of T. Sterry Hunt, who has discussed this subject with 

 great ability, differ very much from the above, though he seems to regard dolomite in its 

 origin as from sediments, for he brings in "the subsequent action of heat." He says, 

 " Dolomites, magnesites and magnesian marls have had their origin in sediments of mag- 

 nesian carbonate of magnesia. These solutions have been produced by the action of 

 bicarbonate of lime upon solutions of sulphate of magnesia, in which case gypsum is a 

 subsidiary product ; or by the decomposition of solutions of sulphate or chlorid of mag- 

 nesium by the water of rivers or springs containing bicarbonate of soda. The subsequent 

 action of heat upon such magnesian sediments, either alone or mingled with carbonate of 

 lime, has changed them into magnesite or dolomite." 



Limestone, especially such as the Eolian, is remarkable for caverns sometimes of great 

 length. We have already expressed an opinion that the cave on Eolus was once the bed 

 of a stream, and have deduced important inferences from the fact. We have the highest 

 authority for the opinion that all caverns in limestone thus originated. " There cannot," 

 says Bischof, "be any other explanation given of the origin of these caves than that they 

 have been hollowed out by water. The fissuring of the strata mugt have preceded this 

 action so that the water might penetrate. But the fissuring of rocks is a very general 

 phenomenon. There is, however, this difference between limestone and slate or sandstone 

 strata: that the constituents of the former are soluble in water and are therefore removed, 

 either wholly, or for the most part, while the constituents of the latter are but partially 

 soluble, or decomposable by water. It is for this reason that caves occur with few excep- 

 tions only, in limestone rocks." 



TALCOID SCHISTS. 



SYNONYMS. 



MICA SLATE AND TALCO-MICACEOUS SLATE (in Berkshire Co., Mass.) : Prof. Chester 

 Dewey's Geological Map of a part of Massachusetts, etc. ; American Journal of Science and 

 Art, I. Series, Vol. VIII, 1824. 



MICA SLATE (Berkshire Co.) : Geological Report of Massachusetts, 1832 ; by Prof. 

 Edward Hitchcock. 



TALCOSE SLATE AND -MiCA SLATE (Berkshire Co.) : Final Geological Map of Mass. 

 1842 ; by Prof. Edward Hitchcock. 



MAGNESIAN SLATE : Prof. E. Emmons 1 works on the Taconic System, 1838-1860. 

 HUDSON RIVER GROUP : Geological Map of New York, 1842. 



