part 2] GEOLOGY OF THE MELDON VALLEYS. 89 



cleavage parallel to its outline. There is a little granular sphene (?). 

 Fragments of this rock occur as inclusions in the 'dark igneous' 

 series. One section of such an inclusion, taken in situ on Sourton 

 Tors, corresponds in all respects with the foregoing description, and 

 in especial presents a Hue example of the flask-shaped inclusions in 

 the quartz. But a new mineral appears, in the form of diverging 

 bundles of acicular anthophvllite, freely developed in the ground- 

 mass, and sometimes even marginally penetrating the quartz- 

 phenocrysts. In consequence of the presence of this new mineral. 

 the rock becomes greenish grey, and macroscopically might be 

 mistaken for an inclusion of shale. 



The i Dark Igneous ' Series. 



It may be well to state the problem which this series presents. 

 C. A. McMahon (5, p. 341) saw in them a series of trachytes and 

 volcanic tuffs. The tuffs he divided into 



' (1) true tuffs, but highly metamorphosed, so that the cementing matrix . , 

 lias bo completely lost all trace of its original fragmentary origin ; it so 

 elosety resembles the base of some quartz-porpliyries and some rhyolites, and 

 has so entirely lost all trace of the agencies by which the change was effected. 

 (2) in two or three cases . . . really igneous flows that have in their passage 

 through, or over, ash-beds caught up numerous fragments of ejected volcanic 

 and sedimentary material.' 



I J. X. Worth (6) considered that the aggloineratic rocks of 

 the series fell under the second of McMahon's classes. 

 Mr. F. P. Mennell recognizes a 



' main band of tuff [which] stretches from Lake, near Bridestowe, to beyond 

 Sticklepath, and, of the numerous well-preserved rock- fragments that it 

 contains, most are of rhyolitic or trachytic character, with some which 

 represent altered andesites.' (Q. J. G. S. vol. lxxii, 1916-17, p. lxxxiii.) 



It is common ground that igneous agglomerates are developed 

 somewhat extensively, presenting the constant feature that, while 

 weathered surfaces clearly indicate their composite nature, most 

 freshly broken surfaces are apparently of compact and uniform 

 texture, except where inclusions of sedimentary rocks exist, and 

 have all the appearance of dyke- or sill-rocks. 



The one hope of solution would appear to lie in the field, and 

 there, while natural exposures are many, few are of themselves 

 very useful. Not until recent years has there been any real 

 opportunity of tracing the relations of the igneous with the 

 sedimentary rocks, and with each other. Now, however, owing to 

 the development of the London Si South-Western Railway quarry, 

 and of the Meldon Valleys Company's quarries, some very useful 

 sections have been available. 



Taken in order from the railway, across the strike, towards the 

 granite the occurrences of the 'dark igneous 1 rocks are as follows: — 



First at LXXY I, S. E. 56. 1 1 ere the sedimentaries are contorted, 

 and it is extremely difficult to ascertain the relationship of the 

 igneous rock to the bedding of the shales. Southward in the 



