1*38.1 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. Ill 



petrographers. I have gone so much into detail in the present case for 

 several reasons. The resultant serpentine isof rare beauty and purity. 

 and is therefore much sought for by collectors in general. The possi- 

 bility of obtaining readily hand specimens which show the abrupt tran- 

 sition within the space of one or two centimeters from pure, unchanged 

 pyroxene to clear, compact serpentine of almost ideal purity, makes 

 the material especially valuable to teachers. I am, moreover, inclined to 

 the belief that future investigation will show very many occurrences of 

 small detached masses of serpentine included in calcareous or schistose 

 rocks to be of similar origin. Indeed, Dr. Hunt,* in arguing against the 

 theory of the intrusive origin of serpentinous rocks, so describes certain 

 localities as to leave almost no doubt that this is the case. He says 

 (p. 435): "In these (the Laurentian gneisses, crystalline limestones, etc.) 

 the serpentine is often disseminated in grains or small irregular masses, 

 giving rise to the varieties of so-called ophicalcite. These imbedded 

 masses of serpentine are sometimes concretionary in aspect and may 

 have a nucleus of white granular pyroxene. They ofteu recall in their 

 arrangement embedded chert or flint, and, like it, sometimes attain large 

 dimensions." Dr. Hunt seems, however, to have regarded the serpen- 

 tine in all these cases as a chemical deposit about the nucleus, instead of 

 a metasomatic product. 



It would seem to the writer, further, that the importance of the move- 

 ments generated by au increase in volume by any large mass of olivine, 

 pyroxene, or other magnesiau silicates in passing into serpentine has 

 not received its full share of attention. May it not be that very many 

 of the slickensided surfaces and local displacements which often prove 

 so misleading when studying rocks undoubtedly stratified, are due 

 wholly or in a large part to this agency ? 



National Museum, May 20, 1888. 



Explanation of Plate xxxi. 

 moxtville serpentine. 

 Nodules of serpentine with nuclei of diopside; Figs. 2 and 3 showing cut and pol- 

 ished surfaces. The light gray is the unchanged diopside; the dark, the secondary 

 serpentine. Fig. 1 shows a nodule from which a portion of the serpentinous crust 

 has been removed, showing the ham-shaped nucleus. This figure shows also on the 

 lower edge the laminated or platy structure mentioned on page 103. 



Explanation of Plate xxxii. 

 montville serpentine. 

 Photomicrographs showing the various stages of transition from diopside to serpen- 

 tine. Fig. 1 shows the direct transition of the gray diopside in the upper right side 

 of hgure into clear compact serpentine. Figs. 2 and 3 show the transition of the 

 white diopside into serpentine, with separation of calcite and the undetermined 

 creamy white mineral mentioned on page 105. In both ligures this mixture of cal- 

 cite and the creamy white mineral is shown by the irregular dark areas in the lower 

 part of the field. Fig. 4 shows the crowding together of the calcite into a compact 

 bundle by the expansion of two granules of serpentinized diopside. 



*Min. Physiology and Physiography. 



