188 Correspondence — Prof. T. G. Bonney. 



miles, it varies much in texture, is generally not conspicuously rich 

 in quartz, and often contains hornblende as well as mica. It exhibits 

 in places a curious streaked or banded structure due to local crushing. 

 This is fully described. The lava flows (with ashes) are porphyrite ; 

 this is generally compact, sometimes glassy in structure, with scat- 

 tered crystals of plagioclase, biotite, hornblende (sometimes augite), 

 a little free quartz, and occasional olivine and apatite. Some peculia- 

 rities in these rocks are described. The intrusive dykes (3) consist 

 of (a) coarse red porphyrite, {h) quartz porphyry, (c) felsite, {d) 

 granite. The author considers that the porphyrite and granite 

 (which are alternately intrusive one in another) belong approxi- 

 mately to the same geological epoch. This is the Lower Old Ked 

 Sandstone ; and as pebbles of the granite as well as of the porphyrite 

 occur in the Lower Carboniferous measures of the region, great 

 denudation must have taken place prior to the latter epoch. The 

 author also describes some basalt dykes which he considers to be of 

 Miocene age. 



coI^I^:Es:po35rID:BI^^c:E. 



DE. ROBEETS ON THE TWT HILL SEEIES. 

 Sir, — I fear that in controversy Dr. Eoberts occasionally allows 

 too much freedom to his imagination. His reply to my criticisms 

 on his communication relating to the conglomerates of the Twt 

 Hill distiict and of Anglesey is in many respects far from accurate. 

 For instance, he draws a plan of the Twt Hill Pit, places a line 

 A B thereon, and states : " Prof. Bonney's section was taken along 

 the line A B" There is nothing in our article (Q. J. G. S. vol. xxxv. 

 p. 321) to warrant this assertion. My section was intended to be 

 drawn roughly in the same line as Prof. Hughes's — but did not 

 include so much, as 1 wished to call particular attention to the bands 

 of conglomerate associated with the arkose (because at the time I 

 considered this important as relating to the age of the rock). One 

 of the bands rather high in the conglomerate did seem to me there to 

 bear a resemblance to the bottom rock, but from subsequent visits 

 I came to the conclusion that I had overestimated the resemblance. 

 The diagram exhibited at the Society was a sketch of the pit (1 

 have the original before me) exhibited to illustrate the same point. 

 From where 1 stood Twt Hill was seen in the position indicated, 

 but I never said the beds dipped under it, and there is nothing in 

 the drawing to make this essential. The outcrop there shown is 

 compatible with their passing on either side. I said the strike 

 seemed E.N.E. Dr. Roberts's diagram makes it the same : how then 

 could my diagram be drawn along A B'i Sections are usually 

 drawn approximately at right angles to the strike of the beds. I 

 did not, however, regard this strike as perfectly accurate, for there 

 were no exposures very well suited for measurement —and with a 

 dozen pupils about one (among whom I believe was Dr. Koberts) 

 asking all kinds of questions, it is difficult to avoid an error of 

 a few degrees. The Cefu Cynryg grits mentioned in my paper of 



