220 J. H. Collins — Cornish Serpentinous Hoclcs. 



in the already formed minerals. The final consolidation took place 

 under conditions of continued strain ; but before it was actually- 

 accomplished minor and subsidiary eruptions took place -which 

 forced ne-w supplies of the granitic material into fissures formed in 

 the previously injected rock, and this fresh material consolidated under 

 conditions somewhat different from those of the first eruptions. 



I think this view meets all the difficulties of the case, and that the 

 intelligent reader will with its aid be able to harmonize all the facts 

 stated above without detailed exposition on my part. 



IV. — On the Geological Histokt of the Coenish Serpentinous 



EOCKS. 

 By J. H. Collins, F.G.S. 

 {Continued from Geol. Mag. Decade III. Vol. III. 1886, p. 366.) 



ITI. EocKS Exhibiting but a Moderate Amount of Serpentinous 



Change. 



BotallacTc Mine. — The dark horublendic slates of the sea-border of 

 the parish of St. Just are known to many geological tourists. They 

 have been well described by the late Mr. J. A. Phillips,^ who regards 

 them as consisting of altered killas ; and the justice of this conclu- 

 sion will not I think be questioned by any who have studied the 

 rocks in situ ; traces of an original lamellar structui-e are visible even 

 in hand-specimens. Phillips (op. cit. p. 322) gives the following 

 analyses of the rock, a being from near the surface, and & from a 

 depth of 130 fathoms, or far below the sea-level ; while c is the 

 analysis of atypical Cornish killas (from Polgooth Mine, 100 fathoms 

 from surface) : — 



^'"«' i cJST- .:?' ) '^'•>' 



Silica 40-22 



Phosphoric acid -66 



Titanic acid -15 



Alumina 24-01 



Ferrous oxide ., 11-27 



Ferric oxide 4-21 



Lime 4-11 



Magnesia 6-52 



Potash 1-67 



Soda 3-57 



99-52 



Sp. Gr 2-95 2-82 2-73 



Evidently a and h are in a highly-altered condition ; but assuming 

 that they were originally of similar composition, approximating to c, 

 it would seem that the deeper-seated rock has been much more 

 altered than the other in the direction of serpentine ; it is also softer, 

 lighter, and more serpentinous in aspect. The following is its 

 appearance under the microscope, as stated by Mr. Phillips or as 



^ Quart Journ. Geol. Society, 1875, toI. xxxi. pp. 319-343. 



