Notices of Memoirs — Gen. McMahon — On Serpentine. 33 



In this connection I may perhaps be allowed to refer again to the 

 occurrence of apatite in slag, alluded to above. This still remains 

 the only recorded case in which that mineral has been observed to 

 crystallize from an artificial silicate magma, analogous to its occur- 

 rence in eruptive rocks. After first noticing it I had opportunity 

 of seeing the slag in question formed almost continuously for many 

 months. It would frequently happen that for days at a time no 

 crystals of apatite could be seen in the cavities, and then again they 

 would reappear in great quantity. Although much attention was 

 given to the subject, nothing could be observed to account for either 

 disappearance or reappearance, no change taking place in composition 

 of the slag, or in conditions of formation and cooling. 



Attempts were made by Dr. Cohen, of Owens College, to obtain 

 apatite in small fusions in the laboratory. Pieces of the slag rich 

 in apatite were melted in crucibles ; similar fusions were made of 

 mixtures of the constituents of the slag in the proportions shown 

 by analysis, and both were subjected to a prolonged " recuit," as 

 in the experiments of Fouque and Levy, but no apatite was ever 

 formed. 



The crystals of Fayalite formed in slags so very rich in iron 

 show very strongly developed cleavages. Rosenbusch alludes to 

 the fact that the cleavages are more distinct in olivine containing 

 much iron, and this observation is fully borne out in thin sections 

 such as those above alluded to. 



1TOTICES OP 1VEE1VCOIBS. 



I. — On the Manufacture of Serpentine in Nature's Workshop. 

 By Major- General C. A. McMahon, F.G.S. 1 



THE rocks from which serpentine is mainly derived by an 

 aqueous process are called peridotites though there are several 

 varieties which receive distinctive names. They are all characterized 

 by the predominance of the mineral peridote or olivine. Some 

 meteorites exhibit a marked affinity with peridotites, containing, 

 like the latter, olivine, rhombic and monoclinic pyroxene, and 

 occasionally some basic felspar. Peridotites are not commonly 

 found on the earth's surface, one reason being that olivine is 

 an unstable mineral that readily absorbs water and passes into 

 serpentine. 



General McMahon detailed the various ways by which water 

 finds its way into minerals ; namely, by cracks ; by planes of 

 cleavage and of " chemical weakness " ; and by capillary flow 

 through the interspaces between molecules. In connection with the 

 latter branch of the subject he gave a sketch of the kinetic theory 

 as applied to solids, and of Boscovich's theory which helped to 

 elucidate the kinetic hypothesis ; and he stated that whether we 

 accept these theories or not, we must give up the idea that the 



1 Abstract of a Discourse delivei - ed before a Meeting of the Western Microscopic 

 Club, London, on the 4th November, 1889. 



DECADE III. VOL. VII. NO. I. 3 



