[I s SERPENTINE OF MONTVILLE, NEW JERSEY. 



Just what ili«' atnoutit of expansion has been can not be estimated abso- 

 lutely, owing to a loss «.f an undeterminable amount of material. From 

 a comparison ot Bpecitic gravities of the two minerals alone it would 

 appear thai this increase was about 29 per cent. This, however, can 

 QOt be considered as more than a rough approximation, since, as shown 

 by the analyses to he noted later, there has been a loss of all the lime 

 and presumably of a part of the silica with smaller amounts of iron ox- 

 ides and alumina.* I am of course aware that as long ago as 1872, J. 

 Leinbergi in describing the highly lustrous and slickenside-like surfaces 

 of certain serpentinous rocks, argued from facts, not necessary to repeat 

 here in full, that their lustrous appearance was due not to movement 

 nor pressure, but to a deposit on the surface of an infinitesimally thin 

 coating of a magnesian silicate. Proof of this was drawn mainly from 

 the fact that the luster appeared not merely on joint surfaces but also 

 on cleavage faces and rough uneven surfaces where there had evidently 

 been no movement. Further, when, as sometimes occurred, a polished 

 serpentinous surface was in contact with limestone, it was the serpen- 

 tinous rock alone that showed the polish while the limestone remained 

 rough and unchanged. The two cases are not, however, exactly par- 

 allel. In the present instance fractured surfaces of the serpentine show 

 frequently a highly lustrous surface, almost resembling- a true cleavage, 

 which may very likely be due to the Chin coating or glazing suggested 

 by Lemberg. The lustrous condition of the exterior of the nodules can 

 not, however, be thus accounted for. Not merely are they polished, 

 but also are grooved and striated like bowlders from glacial drift; fur- 

 ther, this outer portion shows often in places a thinly laminated or 

 platy structure, recalling the platy structure produced in metals by 

 continuous hammering. 1 believe this condition to have been brought 

 about wholly by pressure and motion generated in the mass itself by 

 increase in bulk rather than by orographic movements. The fact that 

 the inclosing dolomite does not show like polished surfaces may be due 

 simply to the fact that during the molecular re-arrangement incident to 

 the conversion of the diopside into serpentine, and its highly hydrated 

 condition, this mineral would naturally be in a couditiou to be molded 

 and scratched by the dolomite, even were not the latter under ordinary 

 circumstances the harder mineral of the two. 



To further show the relationship between the two minerals, samples 

 of both pyroxenes and serpentines were submitted to Mr. Charles Cat- 

 left. of the CT.S. Geological Survey, for analysis. The results are as fol- 

 lows: 



" I ' 1 '- ""i' 1 (Min . Phys., etc., p. 506) has shown that the conversion of olivine 

 into serpentine, it unattended by loss of silica, is attended by augmentation in hulk 

 amounting to :::; per cent. 



ter die coutactbildungen bei Predazza. Neues Jahrbuch, Vol. xxiv. p. 187. 



