286 F. ETJTLEY ON THE SCHISTOSE 



theless Sir Henry was certainly right in his views concerning some 

 of these rocks, which present the appearance of fine sediments in 

 which fragments of scoriaceous and other volcanic rocks are im- 

 bedded ; and it was doubtless from a consideration of these particular 

 rocks that he was led to generalize, and to regard nearly the whole 

 of these schists as ashes. The schistose structure has doubtlsss 

 resulted from the pressure of the superincumbent strata which once 

 covered this area, a circumstance which I pointed out in my official 

 memoir. It may, however, also be partly due, in some cases, to the 

 profuse development of vesicles compressed and elongated in the 

 direction of flow. That these beds are alternations of lava-flows, 

 tuffs, and tufaceous sediments I have no longer any doubt. 



I have learnt the importance of examining only the thinnest 

 possible preparations of rocks of this class, and the importance of 

 expressing a guarded opinion when microscopic rock-sections fail to 

 reveal distinct characters ; I would also here thank Mr. H. W. 

 Bristow for affording me facilities for the re- examination of some 

 of the specimens collected for the Survey. 



The structure of the schistose lavas is in some cases quite clearly 

 shown when very thin sections are examined under the microscope ; 

 but as they have all undergone considerable alteration, it is not easy 

 to speak with certainty about their original mineral constitution. 

 There are numerous little felspar crystals to be seen in them ; and 

 they sometimes lie with their longest axes in definite directions 

 more or less approximating to the direction in which the vesicles are 

 elongated, and which also coincides with the planes of bedding and 

 schistosity. A considerable quantity of viridite is present in most 

 of these rocks ; in some cases it seems to be allied to green earth, 

 and in others to a scaly chlorite-like mineral. I cannot help 

 thinking that these rocks may owe their present fissile character to 

 the elongated vesicles which occur so plentifully in them. 



The felspars are usually very minute, and are often represented 

 by pseudomorphs only. These lavas were most likely similar to 

 some of the highly vesicular basalt lavas erupted at the present day. 



There is an important consideration suggested by the comparatively 

 uniform character of these rocks in the Tavistock, the Saltash, and 

 the Cornish districts ; and it seems to me possible that it may be one 

 which should to some extent influence the position of the boundary 

 line between the Devonian and the Culm series on the west of Dart- 

 moor. In dealing with this question I am fully aware of the value 

 of palseontological evidence, and how such evidence must of neces- 

 sity dominate over any argument based merely upon lithological cha- 

 racters. Unfortunately, in parts of this country, and especially in 

 the critical part now under discussion, fossils are, I believe, few and 

 far between ; and I therefore venture to offer the following remarks in 

 the hope that they may incite palaeontologists to focus their atten- 

 tion for a short time on a small tract of country which seems at 

 present to be doubtful ground. I refer to the tract lying between 

 Tavistock on the south and Lidford on the north. Here the lavas 

 and their associated ashes and tufaceous sediments crop out in strips 



