vol. 51.] series of south denbighshire. 23 



Discussion. 



Prof. Hughes remarked that, although the broad features of the 

 Silurian rocks had long ago been sketched out, there was much to 

 be done in working out the zones in those great masses of stratified 

 rock, each division of which was as large as the whole of the Secon- 

 dary or Tertiary system. Towards this work Mr. Lake's paper 

 was a valuable contribution, and, from his knowledge of that area 

 and of similar sequences elsewhere, he could only confirm Mr. Lake's 

 general conclusions. 



Mr. Hopkinson said that he could corroborate, from his own 

 investigation of the Ludlow rocks in the vicinity of Ludlow, 

 Mr. Lake's correlation of the beds he had described with those of 

 the typical Upper Silurian region. In the Wenlock Shale near 

 Burrington were such forms as Monograptus priodon and Cyrto- 

 graptus MurcJiisoni ; in the Lower Ludlow mudstones just above 

 the Wenlock Limestone at Wigmore, etc., M. colonus occurred in 

 abundance, becoming less numerous as some half-dozen other species 

 of Monograptus gradually came in ; and just below the Aymestry 

 Limestone around Leintwardine and elsewhere, M. leintivardinensis 

 suddenly appeared in swarms. In a bed of shale intercalated in the 

 Aymestry Limestone at Low Bridge near Downton was a peculiar 

 group of dendroid graptolites unknown elsewhere. He had not 

 detected a single graptolite in the Upper Ludlow rocks in this area. 



Dr. H. Hicks said that he had communicated two papers to the 

 Society on some of the beds referred to in this paper ; but they 

 dealt chiefly with the plant-remains in the Pen-y-glog quarry. This 

 paper by Mr. Lake entered far more fully into the subdivisions 

 above the Pen-y-glog Beds ; and he looked upon the discovery of 

 Monograptus leinlwardinensis as of much importance, and as proving 

 clearly that the higher beds of Dinas Bran must be of Upper 

 Ludlow age. 



Mr. Mare congratulated the Author on the completion of a very 

 careful piece of work. He remarked that Monograptus colonus 

 would be doubtless found to range above the Nantglyn Mags into 

 the Leinhvardinensis-beds. Dr. Hicks had many years ago announced 

 to the speaker that he considered that certain beds in the Llan- 

 gollen district were Ludlow, and had exhibited fossils which un- 

 doubtedly corroborated his opinion, but the credit of dividing these 

 beds into zones belonged to the Author. 



Mr. Watts pointed out that the identification of Ludlow rocks 

 in the Long Mountain was originally due to Aveline's mapping for 

 the Geological Survey, although the line there adopted was not quite 

 the same as that indicated by palseontological evidence. Probably 

 the grits below the Leintwardinensis-sl&tes were also present in the 

 Long Mountain. 



The Author remarked that he had not found Monograptus colonus 

 and M. leintivardinensis together in this region ; but he should be 

 inclined to suspect that they might occur together in the beds 

 below the Pen-y-vivod Slates rather than in the Pen-y-vivod Slates 

 themselves. 



