REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 25 



During the last few years numerous discoveries of serpentine have been made, many 

 of which are incontestably derived from peridotites, and show how frequently these rocks 

 must have existed in crystalline schists. The serpentines do not always afford materials 

 for deciding the question as to the origin of the rocks from which they have been 

 derived, but in most cases authors agree in considering these primitive rocks as inter- 

 bedded, and not as injected veins. 1 



Although doubts may exist as to the true structure or mode of occurrence of some of 

 these rocks, it is nevertheless certain that very often true peridotites do not occur in the 

 form of injected veins. Besides, the fact, now recognised, that rocks formerly reputed 

 eruptive are regular intercalations in the crystalline schists, is being continually confirmed 

 by fresh examples. It is unnecessary here to discuss the vexed problem of the manner in 

 which the schists and their associated peridotites may have been formed, though we do 

 not hesitate to admit that the mode of formation must have been very different from that 

 of eruptive rocks. 



From the data which have been collected in the foregoing pages, it is evident 

 that one may admit for the peridotic rocks two modes of origin, but that the question 

 of origin is on the whole to be gathered rather from the behaviour of the rocks in 

 relation to those among which they lie than from mineralogical composition. Unfor- 

 tunately, however, this very important element of the relation of a rock towards those 

 that encircle it is wanting in the case of St. Paul's Rocks. The island stands alone 

 in mid-ocean, and of its connection with other rock-masses we cannot state anything 

 definite. 



If we compare the two hypotheses with respect to the mode of formation of these 

 islets, which are the reasons that plead in favour of the eruption theory ? First, the 

 law of analogy. We know, indeed, that the small oceanic islands are either of coralline 

 or volcanic formation. The observations of Darwin, who was the first to draw atten- 

 tion to this law, have been amply confirmed by geologists who have made these 

 questions their special study. Why should not the Rocks of St. Paul be referred 

 to the same rule ] May not the peridotite of St. Paul be assimilated to the group of 

 crystalline rocks represented by the syenites, diabases, and melaphyres, forming the basis 

 of several volcanic islands in the Atlantic ? * We know for a certainty that these 

 plutonic masses, though not quite presenting the appearance of submerged continental 



quartzites and mica schists, which form the bases of that region, and is covered conformably by a granular mica schist. 

 Cohen remarks that this stratigraphic disposition does not exclude the possibility of the rock being an inclined vein 

 (Lagergang) ; but he adds that the number of peridotites showing an eruptive origin becomes so restricted that one may 

 ask whether there does really exist olivine rocks which possess the characters of eruptive masses. After this remark, 

 the editor of the review places a note which is a doubtful vindication of the eruptive origin of the Iherzolithes, " Fur 

 die Lherzolithe durfte der eruptive Character doch wohl nicht zweifelhaft sein!" 



1 A. Geikie has found a series of beautiful schistose crumpled serpentines intercalated among limestones and 

 schists in the North of Scotland. 



1 Hartung, Geologische Beschreibung der Inseln Madeira und Porto Santo, p. 175. 



D 



