'""TS MK. K. N. WOETH ON THE IGNEOUS CONSTITUENTS OF THE 



tion between them and admitted Dartmoor felsites being that they 

 are of a looser and more open texture, displaying at times a porous 

 character which approaches an irregular vesicularity. They arc 

 apparently the more surface portions of felsitic dykes, or, it may 

 even be, fragments of felsitic lavas ; and, at anj^ rate, they may 

 reasonably be supposed to represent less deeply-seated rocks than 

 the ordinary elvans. The very wide range of characters which 

 elvans assume, even in the same flow, was pointed out in my paper 

 of April last, and need not be enlarged upon here ; and I will only 

 add that approaches to this trachytic or open texture are by no 

 means absent in situ. 



The only examples, then, in the whole series that show any 

 material divergence from existing Devonian rocks of the general 

 felsitic type are (5) the spherulitic felsite ; (13) the greenish rhyolite 

 or liparite ; and (17) the vesicular felsite with horny base — three 

 rocks extremely likely to be found in this connexion, and affording 

 no reason why they should be regarded as an exception to an 

 otherwise universal rule. 



I do not think the fragments generally can be regarded as having 

 been altered to the extent frequently imagined. Structural change 

 has been very slight ; and the chief points to note are the very 

 general modification of colour in sympathy with the ferruginous 

 surroundings (though this is by no means universal), and the 

 somewhat exceptional extent to which kaolinization has proceeded, 

 especially in the andesitic contingents. 



Conditions of Eormation. 



To complete our inquiry we must consider the conditions under 

 which these breccias and conglomerates were formed. That the 

 materials were carried to the places where we now find them by 

 water is clear. It is evident also that this water must have operated 

 over a wide area and with varying force. The current which carried 

 the great blocks of Teignmouth and Dawlish was more powerful 

 than that which distributed the sand and gravel of Heavitree ; but 

 the bulk of the material must have quickly come to rest, or we 

 should have no breccias. At the same time there was enough tri- 

 turation and travel to reduce the softer sedimentary rocks very 

 considerably. These conditions would be fulfilled by high land 

 with short rapid rivers abutting on a shore-line, along which the 

 material which they brought down would speedily be distributed ; 

 and in the present instance I suggest that we have a relic of this 

 high land in what is now Dartmoor. The work done by the existing 

 Dartmoor rivers, in its degree, strikingly resembles that of these 

 ancient Triassic streams ; but the latter, while unquestionably of 

 greater volume, had, in all probability, powerful assistants in a 

 glacial climate on the one hand, and the disturbing effects of vol- 

 canic activity' on the other. Of the existence of volcanic action 

 there is ample evidence. 



Sir Henry de la Beche writes cautiously, but plainly indicates his 



