ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 915 



Summary of Inferences. 



Prom the details, somewhat lengthy I fear, given above, we are, 

 I think, entitled to draw the following conclusions : — 



(1) That the serpentine of the Lizard was originally an intrusive 

 igneous rock. 



(2) That its intrusion was posterior to the metamorphism of the 

 hornblende schist, the fragments of that rock included in it not dif- 

 fering materially from the main mass, though, of course, a little 

 more altered. 



(3) That on the western coast the serpentine has been broken 

 through by several granite dykes. 



(4) That on the eastern coast it has been broken through by the 

 following igneous rocks : — 



(«) Gabbros, which at Coverack Cove are certainly of two 

 very distinct dates ; and the later are most probably of the age of 

 the great gabbro mass of Crousa Down. Similarity of mineral 

 character would lead me to regard all the east-coast outbreaks as 

 far as the Balk as of the same date, viz. that of the Crousa- 

 Down massif. 



(b) Certain dark trap dykes found, like the gabbros, only on 

 the east coast, having much the same range, and the latest of all 

 in date. 



(»5) That the gabbros appear liable to three forms of mineral 

 change. 



(a) The gradual conversion of their felspar into a microcrys- 

 talline saussuritic mineral. 



(6) The conversion o^ their diallage into hornblende by pseu- 

 domorphism, or rather by a recrystallization, not generally by 

 paramorphism, and that in certain cases some olivine disappears 

 in this process. 



(c) The conversion, more or less complete, of the olivine into 

 serpentine, in which case the diallage appears little changed. 



(6) That these trap dykes were very probably once all dolerites 

 or basalts, and that the hornblende, which undoubtedly characterizes 

 many of them, is a secondary product due, as above, to metamor- 

 phism of the original pyroxenic constituent. 



(7) That the metamorphism of the serpentine was probably com- 

 plete before the intrusion of any of the above rocks. 



(8) That the serpentinous aspect of a rock is often rather illusory, 

 being due to the presence of an extremely small proportion of that 

 mineral ; hence that statements about the conversion of ordinary 

 pyroxenic or hornblendic rocks into serpentine require confirmation 

 from microscopic examination. 



Microscopic Examination of the Serpentine. 



With the view of ascertaining, if possible, the original character of 

 the rock which now constitutes the serpentine of the Lizard, I have 



