138 METAMORPHIC BOCKS 



morphie rocks ricli in lime-magnesian pyroxenes or amphiboles, 

 as malacolite and tremolite. To sucli an origin are to be referred 

 the serpentinous limestones of Essex County, New York; Easton, 

 Pennsylvania, and Montville, New Jersey. In the last-named 

 instance the original rock was coarsely crystalline dolomitic 

 limestone containing numerous nodular masses of white pyroxene 

 (malacolite). Under this metasomatic process the pyroxenes 

 yielded up their calcium, which recrystallized as caleite, while 

 the silica and magnesia, combined with some 13% of water, re- 

 mained as a beautiful green and yellow serpentine. The trans- 

 formation was accompanied by a considerable increase in bulk, 

 whereby the exterior of the nodules, pressed against the rough 

 walls of the enclosing rock, became scratched and polished like 

 boulders from the glacial drift, or the entire mass even took on a 

 platy, schistose structure. Figure 8, from a specimen in the 

 National Museum, illustrates a transitional phase of this change, 

 the interior rounded mass of a gray color being of still unaltered 

 pyroxene, while the dark material forming the exterior shell, 

 or traversing the gray in fine thread-like veins, is the secondary 

 serpentine. In a like manner in all probability originated the 

 peculiar structure imitative of animal organisms known as 

 Eozoon Canadense?- 



The conversion of a limestone into a dolomite is believed to 

 have been brought about by a somewhat similar process. Indeed 

 it is doubtful if this last-named rock is ever a product of direct 

 sedimentation or precipitation. Although sea-water contains 

 from three to four times as much magnesia as lime, evidence is 

 wanting to show that the material is ever secreted in appre- 

 ciable quantities by marine animals, and hence the sedimentary 

 deposits must be correspondingly lacking in this constituent. It 

 has been argued by Beaumont and others that through a process 

 of partial molecular replacement (metasomatosis) pre-existing 

 limestones were converted into dolomites, the process consisting 

 in the replacement of every other molecule of calcium carbonate 

 by one of the magnesium carbonate. As the dolomite molecule 

 is the more dense of the two, such replacement, in any given 

 limestone bed, must result in a contraction amounting to some 



^See On tlie Serpentine of Montville, New Jersey, Proe. U. S. National 

 Museum, Vol. XI, 1888; Notes on the Serpentinous Rocks of Essex County, 

 New York, etc., ibid., Yol. XII, 1889; and On the OpMolite of Thurman, 

 Warren County, New York, Am. Jour, of Science, Vol. XXXVII, 1889. 



