J. H. Collins — Cor)mli Serpentinous Eocks. 225 



Bahhacomhe Bay. — This, like the last, is intrusive in Devonian 

 rocks. " There are here two principal varieties of dolerite — one 

 rather fine-grained and of a uniform grey colour ; the other of 

 a lighter shade and porphyritic texture, having conspicuous crystnls 

 of felspar scattered through it. Both varieties are highly altered ; 

 the secondary products are serpentine and calcite."^ Much of the 

 original augite of this rock seems to have been converted into 

 hornblende, as in cases already mentioned. No analysis of the rock 

 has been made. 



Serpentinous change has been observed in a great many other 

 localities within the two western coimties, but probably these will 

 suffice as illustrations. 



The methods of the changes which have taken place in these various 

 rocks have been already adverted to (Geol. Mag. 1885, Dec. III. Vol. 

 II. pp. 298-302). It would occupy too much space to discuss these 

 changes completely, as the inquiry is complicated by the great diversity 

 of composition indicated by the various analyses. In comparing 

 these, the very variable amount of iron in its two states of oxidation, 

 of alumina, and of lime, arising from the jjresence of included 

 crystals and grains of various substances, cannot fail to be noticed ; 

 and of course these greatly varying proportions aifect those of the 

 water and silica, and especially those of the magnesia. The diversity 

 of composition, however, is more apparent than real, and is owing 

 in some instances to the irregularly porphyritic character of the 

 original rock, and in others to subsequent infiltration and crystalliza- 

 tion. Thus the alumina may be generally traced to the presence 

 of still undecomposed crystals of felspar, or else to kaolin resulting 

 from such ; to augite, or hornblende ; the iron to magnetite or to 

 flocculent masses of peroxide of iron; and the lime either to felspar 

 or else to patches or veins of calcite. Take away these, and the 

 serpentinous ground-mass will often be found to consist of nearly 

 pure serpentine. 



Conclusion. 



I have thus shown that serpentinous change is extensively met 

 with in both stratified and intrusive rocks in many parts of the two 

 w^estern counties of England. The rocks sometimes change as a 

 whole — as at Botallack and Clicker Tor — or in certain of their foliar 

 — as at Cant Hill— or thin films of serpentine are produced, bounding 

 the numerous joints which divide the rock into angular fragments, 

 still purer serpentine, with or without asbestiform matter, being 

 deposited wherever the joints are sufficiently open — as at Terras. 

 Crystals of augite, olivite and hornblende all appear to have been 

 frequently changed into serpentine — with or without intermediate 

 changes — while felspar crystals, if present, are either converted into 

 saussurite, or else are more or less completely kaolinized. In the 

 case of the hornblende crystals, the change begins by loss of 

 dichroism. Then, as in the case of augite also, the crystal breaks 

 up into a fibrous mass of acicular crystals ; and finally amorphous 



^ Allport, ibid. 



DECADE III. VOL. IV. — NO. V. 15 



