AXXIYERSAUT ADDRESS OF THE PEESIDEXT. Ixxi 



signifLcance of similar phenomena ^hich he had obseiTed in the Eifel 

 and in the Tiyarais. The ordinary lavas are, as we well know, far 

 more easily fusible than this inclnded mineral, and especially than 

 those paler varieties constituting the true chrysolite, which contain 

 a larger amount of magnesia in relation to the protoxide of iron. 

 And although it is abundantly proved that crystallized minerals very 

 closely comparable with olivine or peridote have been produced by 

 direct fusion in some of our blast-furnace slags, it appears certain 

 that in many of the streams of lava and of doleritic rock this mineral 

 had formed a solidified mass before it was broken up and floated 

 away by the less refractory material in which we find it imbedded. 



M. Daubre'e proceeds to infer that, although comparatively re- 

 stricted at the surface, rocks of the peridote class would at a cer- 

 tain depth predominate ; and seeing the nature of the samples 

 which they appear to yield us from other parts of the planetary 

 system, he would comprise them, with the serpentines, in a special 

 division as the jperidotic famihj, or cosmical rodcs. 



A few distinguishing characters between these and other silicated 

 rocks deserve to be noted : — 



1. Peridote presents us with the most basic type known, whether 

 in meteorites or in eruptive rocks, and must in this respect, and for 

 its simplicity and definiteness, take precedence of the pyroxenic and 

 other types, which have been proposed by Bunsen and Durocher. 



2. The facility with which both peridote and enstatite crystallize 

 after simple fusion contrasts remarkably with the behaviour of the 

 aluminous silicates, and especially the felspar group. 



3. The decided superiority in density of this type over that of the 

 basalts, as of the latter over the granites ifcc, as seen by the follow- 

 ing table of specific gravities : — 



Granite 2-64 to 2-76. 



Diabase 2-m to 2-88. 



Basalt 2-9 to 3-1. 



Enstatite 3-303. 



LherzoHte 3-25 to 3-33. 



Peridote 3-33 to 3-44. 



Hence it would seem probable that, in theii' origin, these different 

 substances would have arranged themselves in the above order, 

 and that the high density of peridote would explain the depth 

 of its position, below the granitic covering, in the crust of the 

 earth. 



Passing to the examination of serpentine, a rock analogous in re- 

 spect of its silicate of magnesia, but differing from peridote and 

 Iherzolite in being hydrated, comparatively infusible, and without 

 distinct crystallization, specimens fi^om several localities (Snarum in 

 Norway, Zoblitz in Saxony, Monte Ferrato in Tuscany, (tc.) were 

 melted down. The result was in every case a more or less clearly 

 crystalline mixture of peridote and enstatite, the former being more 

 abundant when additional magnesia was placed in the crucible. 



