1888.'] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 107 



lions from the line of contact show most beautifully the direct transi- 

 tion of the diopside into serpentine, as illustrated in figs. 1 and 2, PI. 

 xxxn, engraved directly from photomicrographs by Mr. T. W. Smillie. 

 Each crystal is bordered by a narrow fringe of parallel lying serpentine 

 fibers standing at right angles with the crystal itself, after the manner 

 so well known in serpentine pseudomorphs as to need no further de- 

 scription. Fracture and cleavage lines have given way to irregular 

 canals of serpentinous matter, and every gradation can be traced in a 

 single section from the fresh diopside to pure, compact serpentine. 

 Secondary minerals (other than serpentine) are surprisingly rare in the 

 sections at baud. In all those from the larger masses the gray pyrox- 

 ene is seeu passing directly into pure serpentine w-thout a trace of ad- 

 mixtures of free calcite, silica, or iron oxides, all having evidently been 

 removed as fast as formed. (See fig. 1.) In some instances where the 

 pyroxeue occurs in nodules but a few millimeters in diameter inbedded 

 in the dolomite the transition into serpentiue is made noticeable by the 

 formation of a reddish zone of iron oxides. Sections from the white 

 pyroxene differ in that they still show in the form of small calcite gran- 

 ules the excess of lime set free during the process of transformation. 

 (See figs, li and 3.) As both pyroxenes contain essentially the same 

 percentages of CaO such differences can scarcely be expected to prove 

 constant on further investigation. Even where the alteration is com- 

 plete and no trace of the original pyroxene remains, the origin of the 

 serpentiue by hydration of some magnesiau silicate is made at once 

 apparent by such appearance as shown in fig. 4, PI. xxxn. Here the 

 gradual increase in bulk of two adjacent granules of the pyroxeue has 

 crowded the calcite grains lying between them into a compact bun- 

 dle, while immediately beyond they spread out into broad fan shaped 

 areas, giving rise to a pseudo fluidal structure. These evidences of ex- 

 pansion suggest a possible explanation of the slickensided surfaces s^en 

 on all the nodules, aud which indeed are common to serpeutinous rocks 

 wherever found, aud emphasize the suggestion made oy Diller* and 

 others to the effect that they are due chiefiy, if not wholly, to motion 

 generated in the mass of rock by increase in volume. Take the case 

 of an original nodule of the pyroxene imbedded in the dolomite. As 

 hydration goes on more space is demanded aud the serpeutinous matter 

 is pushed out into every available nook aud crevice. Possibly through 

 force of expansion fractures will be formed in the inclosing rock, and 

 as the serpentiue is pushed out gradually into these spaces it comes in 

 contact with the rough walls of the inclosing rock, and is grooved and 

 polished in direct proportion to the amount of movement aud the hard- 

 ness and resistauce of the material of which it is composed. The ex- 

 treme compactness of the serpentiue is doubtless due largely to the 

 resistauce to expansion offered by the dolomite in which it is imbedded. 



* Geology of Lassen's Peak District, California. Eighth Annual Report U. S. Geo- 

 logical Survey, 183b-'67. MSS. notes. 



Proc. X. M. SS— s Jrrr.*,,n*. 



