Vol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 359 



That the results ohtaiued at Cock's Tor differed from those which 

 followed the metarnorphism of the Sourton Tors-Meldon ash-beds 

 was probably due to some slight difference in the chemical and 

 mineralogical character of the lavas that supplied the fine-grained 

 interstitial material at Sourton Tors and Meldon, and the volcanic 

 dust of Cock's Tor. There may also have been some slight difference 

 in the circumstances that governed the metamorphism in the two 

 cases. There may have been more water, or the water may have 

 been more highly charged with acid, in the one locality than in the 

 other ; or there may have been greater heat, or some other factor, 

 present in the one case that was absent in the other. 



However this may have been, there is no escape from the fact that 

 the Cock's Tor rocks are now composed of augite and felspar, and 

 that these beds on their weathered surface give evidence of having 

 once been laminated deposits. As these beds occur in an area that 

 abounds in volcanic ash, and as this portion of the area exhibits 

 contact-metamorphism of a pronounced kind, I think the most 

 reasonable and probable conclusion to form is that they are tuffs 

 which have been altered by contact-metamorphism. 



In my first paper on the Lizard rocks I showed x that the horn- 

 blende of the schists was a secondary product due to the alteration 

 of augite by aqueous agencies, and that examples of this change 

 in all its stages can be seen in thin slices of these rocks when 

 examined under the microscope ; also that the rocks give 

 " abundant evidence of the presence and action of water." " The 

 competence," I added, " of this agent, aided by heat and pressure, 

 to bring about great mineralogical and structural changes, can 

 hardly be doubted. Indeed, the Lizard rocks have been penetrated 

 by and have yielded to the action of aqueous influences so completely 

 that they may almost be said to have been stewed in water." 



The Cock's Tor rocks exhibit the same changes and the same 

 agencies, only in a lesser degree. Here and there small portions of 

 almost colourless augite-crystals (I' speak of their appearance in 

 transmitted light under the microscope) have been converted into a 

 strongly pleochroic hluish-green hornblende — the beginning of those 

 changes that would in time, and under favourable circumstances, 

 have converted the metamorphosed ash-beds of Cock's Tor into 

 hornblende-schists, indistinguishable from the Lizard hornblende- 

 schists of tufaceous origin. 



That the changes set up in the Lizard rocks have proceeded 

 further than the changes begun in those of Cock's Tor may be 

 owing to the circumstance that the rocks of the Lizard formed the 

 roots of an ancient mountain-range, and were presumably more 

 exposed than the Cock's Tor beds to the heat and pressure that 

 gave greater potency to the aqueous agents at work : for the Cock's 

 Tor beds, it is to be presumed, lay nearer the surface, and were 

 consequently less involved in the pangs and throes of mountain- 

 making. 



1 Quart. Journ. Geol. Soc. vol. xlv. (18S9) pp. 522-527. 



