ANNIVERSARY ADDRESS OF THE PRESIDENT. I 53 



there and in Wales and the Lake country, but still more strikingly 

 by innumerable sections where thin intcrstratifications of tine tuff 

 or volcanic breccia occur among the ordinary sedimentary strata, 

 and are sometimes crowded with Bala fossils. Some interesting 

 illustrations of this feature are to be seen in the Enniscorthy 

 district, where layers of fine felsitic tuff, sometimes less than an 

 inch in thickness, lie among the shales. In some of the tuffs the 

 lapilli are fragments of trachytic or andesitic rocks. 



The most striking example of pyroelastic material I have met with 

 is situated far to the south in County Waterford, close to Dunhill 

 Bridge, where a remarkable group of fine volcanic breccias and grits 

 occurs. These strata consist of coarser and finer detritus, enclosing 

 angular fragments of felsites and grey and black shale. The felsites 

 vary in texture, some of them presenting beautiful flow-structure. 

 The stones are stuck at random through each bed, the largest being 

 often at the bottom. The beds of breccia vary from a few inches to 

 a foot or more in thickness. There can, I think, be little doubt that 

 each of these breccia-bands points to a single volcanic explosion, 

 whereby felsitic fragments were thrown out mingled with pieces of 

 the Silurian strata, through which the vents were drilled. In a 

 vertical thickness of some fifty feet of rock there must thus be a, 

 record of ten or twelve such explosions. 



Nearer the active vents the fragmental deposits become, as usual, 

 coarser and thicker. But I have not yet met with any thick masses 

 of tuff like those of North Wales. So far as my examination has 

 gone the tuffs are mainly felsitic. The so-called " greenstone- ash " 

 of the Survey maps is certainly in many cases not a true tuff. This 

 term was proposed by Jukes for certain apple-green to olive-brown 

 flaky fissile rocks only found " in association with masses of green- 

 stone." * Some years ago I had occasion to make a series of 

 traverses in Wicklow and Wexford, and then convinced myself that 

 in that part of the country the " greenstone-ashes " were probably 

 crushed bands of the basic sills. Dr. Hatch has proved this to be 

 their origin from a series of microscopic slides prepared from speci- 

 mens collected by himself on the ground t. In other cases the 

 *' greenstone-ashes " seem to be excessively-cleaved or sheared 

 felsites which have acquired a soapy feel and a dull green colour ; 

 but they also do include true tuffs. Thus, in one instance, at Bally- 

 voyle cross-roads, in the south of County Waterford, a " greenstone- 



* Explanation of Sheets 129, 130, p. 13 (1869). 

 t Explanation of Sheets 138, 139 



