IGNEOUS ROCKS OF SOUTH-EAST CORNWALL. 23 



There remains now only the Serpentine or Picrite of Clicker 

 Tor. The intrusive character of this rock is clear, but the only' 

 fact we have to date it by is that it rises in Devonian rocks, 

 satisfactorily proved to be such by the occurrence within a short 

 distance, of thin bands of interstratified Devonian lava. The 

 Clicker Tor rock was originally an olivine-dolerite, the olivine in 

 which is now chiefly converted into serpentine. Felspar is also 

 indicated, with plates of unaltered augite and some magnetite. 

 The only rocks in the more immediate locality that exhibit any 

 trace of relationship to this are intrusive masses at Eock and 

 Yealmpton, near Plymouth, which contain serpentine, in part a 

 result of the alteration of olivine. The main point to be settled 

 is whether the difference between this rock and the gabbros is 

 due to the different lines which metamorphism has pursued (and 

 it seems to be conclusively proved, as Mr. AUport has shown, with 

 regard to the hornblendic rocks of Penzance, that proximity to 

 the granite has resulted in hornblendic change*) ; or whether 

 the presence of olivine does not indicate not only a marked 

 difference in original composition but also in date. The presence 

 of mica in the doleritic group of Saltash appears very clearly to 

 differentiate that series. 



If I have succeeded in making my points clear, it will be 

 gathered that the igneous rocks of South-East Cornwall do afford 

 some clues to the ages of the strata with which they are associ- 

 ated. The granites, which, next to the Devonian volcanic series, 

 are the most definitely dated, are however of practically no 

 chronological value to us, because of the high relative antiquity 

 of the rocks through which they rise. The other intrusive rocks, 

 when their relations are better understood, wiU in all probabiKty 

 afford us some aid, for they seem to give evidence of recurring 

 periods of igneous activity. 



Our chief reliance must, however, be upon the volcanic 

 series first described. Wherever they are found we may, I believe, 

 identify the rocks with which they are interstratified as Devonian, 

 — indeed Middle Devonian ; and in many cases we shall be 

 able to reason with reasonable certainty from the datum thus 

 given, to the age of he non-Devonian rocks of the immediate 



* Qmot. Jour. Geo. Soc, Vol. XXXII., p. 425. 



