342 Mr Forbcs's Physical Notices of the Bay of Naples, 



particular substances admitted to exist on the island, and with 

 which it must be in connection. 



Serpentine at present is one of the most undetermined 

 members of our geological arrangement. Dr Macculloch, 

 in his work on the classification of rocks, has admitted the 

 difficulties attendant on its phenomena, and has indeed al- 

 tered in the last sheet, the views which he had given on the 

 subject in the middle of the book ; but its connection with 

 trap rocks, in some situations at least, is undoubted. He 

 mentions two instances in which a vein of greenstone, rising 

 through secondary limestone, (qu. of the magnesian varie- 

 ty .'^) on coming into contact with that rock, assumed the 

 characters of perfect serpentine, containing the characteris- 

 tic minerals of asbestus and steatite, with talc on its margin 

 next the limestone, and the vein exhibited the most perfect 

 gradation from the trap into the serpentine.* He has also 

 observed the intimate connection of serpentine with hornblende 

 rock in the primitive strata. I have myself studied one of the 

 most important phenomena connected with this question, in 

 Inch Colm, an island in the Forth, where secondary greenstone, 

 secondary syenite, and secondary serpentine, containing well 

 defined asbestus and talcose matter, are most instructively com- 

 bined. A dyke of serpentine in Forfarshire, led Mr Lyell to 

 the conclusion of the igneous origin of this rock, -|- and Dr 

 Boue has observed similar facts on the continent ; J he has 

 distinctly arranged serpentine in his classification, under ". un- 

 stratified crystalline or igneous rocks." || At the present day, 

 little is required to enforce the similiarity of substances con- 

 nected with trap rocks and those of volcanos. But lest it 

 should be thought that the union of serpentine with true vol- 

 canic rocks is not a necessary consequence from the analogy of 

 trap, we may add, that Sir George Mackenzie, in his work on 

 Iceland, gives an account of a volcanic amygdaloid, in the 

 mountain of Akkrefell, traversed by veins of serpentine of 

 more than a yard in thickness. Ferber considered, and it 

 would appear not without great reason, so far as external cha- 



• Classification of Rocks, p. 245. 

 •f Geological Transactions, 

 t Daubeny on Volcanos, p. 430. 

 II Edin. Phil, Journ. xiii. 132. 



i 



