Vol, 50.] IGNEOUS ORIGIN ON DARTAIOOR. 355 



radiating fibrous crystals. Prolonged heating in concentrated hydro- 

 chloric acid has no effect upon it. It polarizes well, and, when the 

 slice is sufficiently thick, exhibits pleochroism. 



The original base has been altered into a serpentinous product, 

 which is probably related to pseudophite. 



These interesting rocks of Cock's Tor remind me very forcibly of 

 some of the hornblende-schists of the Lizard. I allude to that 

 variety of the Lizard hornblende-schists which is supposed to have 

 been originally a volcanic tuff. This origin was attributed to the 

 Lizard schists by De la Beche long ago, and was adopted in my 

 first paper on the Lizard rocks. 1 In the joint paper subsequently 

 written by Prof. Bonney and myself, 2 we stated that the rocks of 

 the Hornblendic Group "must originally have been of igneous origin ; 

 the more massive may represent altered basaltic lavas, the more 

 handed altered tuffs of similar composition." 3 And whilst the fluxion 

 hypothesis was invoked to explain the present foliated schistose 

 character of former lava-flows, we were of opinion that "the. possi- 

 bility of some portions [of the Lizard hornblende-schists] having 

 resulted from the alteration of a stratified basic ash must not be left 

 out of sight.'' 4 



My first paper showed that the Lizard hornblende-schists contain 

 a colourless augite, and I expressed the opinion that the hornblende 

 which enters so largely into the composition of these schists was a 

 secondary product after augite. In these Lizard rocks the derivation 

 of the hornblende from the augite could be clearly demonstrated by 

 microscopical evidence. 



In the rocks on the flank of Cock's Tor described in these pages, 

 we have, it seems to me, hornblende-schists, similar to those of the 

 Lizard, in an early stage of their development. In both we find 

 almost colourless augite set in a felspathic base, or in felspar that 

 plays the part of a base. 3 In the Lizard rocks the augite has nearly- 

 all been converted into hornblende ; in the rocks of Cock's Tor this 

 process has only just commenced, and we can see it in its first stages. 



The microscopical evidence demonstrated clearly that aqueous 

 agencies were the cause of the change of the augite into hornblende 

 in the Lizard schists. In the case of the rocks of Cock's Tor, I 

 think the evidence points to the same conclusion. 



An original fragmentary origin was predicated for the Lizard 

 hornblende-schists on the grounds that they exhibited structures 

 suggestive of stratification and even of ' false bedding,' and that 

 their chemical analysis and mineralogical composition indicated 

 an igneous origin. 6 In the case of the Cock's Tor rocks, similar 

 and even stronger grounds exist for considering them to be highly 

 altered ash-beds. Their mode of occurrence is not only suggestive of 

 bedding, but they bear on their face evidence of original lamination. 



1 Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 519. 



2 Op. tit. vol. xlvii. (1891) p. 464. 3 Ibid. p. 478. 



1 Ibid. pp. 480, 497. s Op. cit. vol. xlv. (1889) p. 521, 



6 Op. cit. vol. xlvii. (1891) p. 478. 



