REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 23 



serpentines derived from the decomposition of peridotites, such as are found in the Lizard 

 district, are intrusive. 1 The same geologist, who has recently made a study of the 

 Iherzolites 2 of the Ariege states that these rocks must have been intruded through 

 crystalline limestone. The observations of Zirkel on peridotic rocks from Iceland deserve 

 to be noticed here. 3 He points out that although the basalts and anamesites of Iceland 

 are almost wanting in olivine ; yet on the north coast of the island, near Melstadr, 

 Hnauser, and Hofsos, thick isolated layers of rocks, composed almost exclusively of 

 olivine, are found intercalated in the lava ; the amount of augite present in this rock 

 being so small that the mass may be considered as formed entirely of olivine. 

 Naumann 4 adds that the fact proves that this mass of olivine rock was ejected in a fused 

 condition, and spread out in layers while yet fluid. 



Another variety of peridotic rock, described by Tschermak under the name of picrite, 

 was found by him in Moravia and Austrian Silesia, where it appeared in the Neocomian. 

 In this rock, olivine forms about one-half of the whole mass, with the addition of diallage, 

 hornblende, and mica. These picrites have sometimes the habitus of gabbro. Not only 

 does the study of their behaviour in the field indicate their igneous origin, but the same 

 inference may be drawn also from their microstructure. In the typical specimens collected 

 by Tschermak, the presence of a vitreous base can be detected. As for the palaeopicrites 5 

 discovered by Sandberger in older formations, it appears from his observations and those 

 of G umbel that they also are eruptive. Among the experimental proofs which may 

 still further be invoked to prove the igneous origin of peridotic rocks are Daubree's 

 celebrated experiments, in which he reproduced Iherzolite artificially by the dry fusion of 

 chondritic meteorites, thus proving, in the most conclusive manner, that nature may 

 have adopted the same process in the formation of rocks, having a like mineralogical 

 constitution. 



But while the peridotites above mentioned are eruptive, it is none the less true 

 that many masses of olivine rock present characters from which an igneous origin 

 cannot be demonstrated. 6 The grounds for this assertion may best be shown by a brief 



ordinary light, to be composed of a mass of homogeneous olivine, but in polarised light was seen to be composed of 

 granules of irregular size, and much larger than those of the olivine of St PauL With this exception the other 

 microscopical characters are the same in both rocks: the fissures, more or less regular, marked by black lines, intense 

 chromatic polarisation of the olivine, roughness of its surface, &c. &c. The sections of chromic iron in dunite are larger 

 than those in the specimens from St Paul, but in other respects they present the same features. 



1 Bonney, On the Serpentine and associated Rocks of the Lizard District, Quart Jour. GeoL Soc., voL nrriii. 

 pp. 884-924. 



* Bonney, The Lherzolite of the Ariege, GeoL Mag, Decade ii, voL iv. pp. 59-64. 



3 Zirkel und Preyer, Eeise nach Island, p. 292. 



4 Naumann, Lehrbnch der Geognosie, vol. iii. p. 365. 



s See Oebbeke, Ein Beitrag zur Kenntniss des Palaeopikrits, 1877. 



6 Rosenbusch, Massige Gesteine, p. 526. This author has felt the inconvenience of grouping together all the 

 peridotites, and confesses that analogy of composition cannot extend in a general way to analogy of origin ; he admits 

 that the igneous origin of many peridotites is not proved, and that he should not, therefore, have included in his 

 manual on the massive rocks some peridotites which appear as regular intercalations among crystalline schists. 



