204 SERPENTINE. 



its analogy in the trap rocks which have been fused 

 in situ. When in masses involved in strata, that ana- 

 logy is found in the similar limestones. And as the 

 fluid intruding trap of veins is in no wise different from 

 that fused in situ, so is explained the mineral identity 

 of serpentine in these different cases ; while, of this 

 condition, its connexion with, and transitions into trap, 

 leave as little doubt as does the truly venous form. 

 And thus also is explained its claim on both classes^ 

 and its decidedly independent connexion with each ; 

 being exactly what occurs in the porphyries. Had I 

 ranked it with the unstratified substances rather than 

 the stratified, I should be justified by the usage re- 

 specting trap, possessing, when in situ, the same double 

 claims. It is evident that the presence of calcareous 

 veins is no objection to these views, as they are of 

 posterior and aqueous origin ; while it confirms them to 

 note, that the minerals which it contains, such as py- 

 rope, diallage, and hornblende, are of igneous origin. 

 The magnetic influence of Serpentine has been no- 

 ticed by Humboldt, as if it was a rare and peculiar 

 phenomenon. But it is not thus limited; being very 

 common in trap, occurring also in granite, and, as far as I 

 have experienced, in almost every rock which contains 

 iron in a certain form and quantity. In Unst, where 

 the Chroma! abounds in the Serpentine, this influence 

 is easily explained. If these disturbances of the needle, 

 produced by the proximity of certain rocks, have been 

 observed, few have examined the quantity of the effect, 

 or the positions of the poles and planes of no action. 

 Nor,excepting Saussure, have Geologists remarked those 

 deviations of the needle which are produced by large 

 masses acting at greater distances ; though they could 

 scarcely be overlooked by any one, who, either from 

 navigating by the needle near to land, or from habits 



