116 THE GEOLOGY OF THE MEL130M TALLEYS. [vol. lxxv, 



strata with an average dip of 50°, bounded on both sides by the 

 * dark igneous ' rocks, and the fact that the two districts were but 

 a few miles apart, made the speaker doubt whether the succession 

 was really a normal one. However, the geology of Devon is so 

 extremely complex, that the two districts may be quite different in 

 structure. 



Dr. J. W. Evans welcomed this valuable contribution from 

 one of the all-too-small band of local workers in Devon. He 

 commented on the excellence of the microphotographs, and hoped 

 that the very convenient practice of placing a scale of length on 

 each would be followed by other authors. He was certainly under 

 the impression, from his personal observations, that there were 

 ash-becls interstratified with the slates near Belstone, which he had 

 visited along with Dr. Holmes ; but the Author appeared to have 

 made out a strong case for the intrusive character of the clastic 

 igneous rocks described by him. The speaker suggested that the 

 relations between the fragmentary and the fluidal igneous rocks in 

 this case might be explained by earth-movements giving rise to 

 shatter-belts involving recently-consolidated intrusions, followed 

 by later intrusions which penetrated into the interstices of the 

 fragmental material. He was glad that the Author had given a 

 description of a recent example of the extraordinarily-effective 

 action of streams on occasions of catastrophic rainfall. Such 

 occurrences, though they might be separated by long intervals of 

 time, were part of the normal processes of Nature, and probably 

 accounted for the greater portion of fluviatile erosion. The 

 President had recently laid stress on this fact, which was as a rule 

 not sufficiently realized. 



Sir Dotjglas Mawsox, referring to the Author's suggestion that 

 the re-entrant angles in the outline of a cross-section of chiastolite 

 embedded in a fine-grained base (shown in one of the microphoto- 

 graphs) appeared to indicate twinning, remarked that a similar 

 feature is exhibited in the case of the wonderfully-developed 

 crystals occurring at the famous locality of Mount Howden, in 

 South Australia. There the individuals, which are some inches 

 long, offer special facilities for examination in detail, but have 

 proved to be simple and not twinned. An investigation had led 

 him to regard, as the chief influence favouring such a development, 

 the existence of conditions exceptionally suitable for very rapid 

 growth of the crystals. 



Dr. H. H. Thomas agreed with the previous speaker that 

 the crystals of chiastolite were simple crystals, despite their 

 curious cruciform habit. The crystals appeared to have developed 

 both prismatic and pinacoidal faces up to a certain point, after 

 which all deposition appears to have taken place upon the prism- 

 faces, thus bringing about the suppression of the pinacoids and 

 the formation of those re-entrant angles that give the superficial 

 appearance of twinning. 



Prof. W. W. Watts stated, with reference to the 'intrusive tuffs' 

 mentioned bv the President, that one set of these occurrences was 



