ANNIVERSARY ADDRESS OF THE PRESIDENT. 99 



dark shales and grits of the Lower-Siluriau (Areiiig) series. 

 Some of the most persistent bands among them are conglo- 

 merates, which differ from each other in composition, but most 

 of which consist largely of fragments of various igneous rocks. 

 Some of the coarser masses might be termed agglomerates, for they 

 show little or no trace of bedding, and are essentially made up of 

 blocks of volcanic material. There are abundant beds of grit, some- 

 times pebbly or finely conglomeratic, alternating with tuffs and with 

 bands of more ordinary sediment. Courses of purple shale and 

 sandstone, green shale, and dark grey sandy shale occasionally occur 

 to mark pauses in the volcanic explosions. Perhaps the most striking 

 feature in the pyroclastic materials is the great abundance of very 

 fine compact pale tuffs (hiillefiintas of some writers), sometimes 

 thinly laminated, sometimes occurring in ribbon-like bands, each 

 of which presents internally a close-grained, almost felsitic or flinty 

 texture. 



A cursory examination of the contents of the conglomerates, 

 breccias, and grits shows them to consist largely of dijSterent felsites, 

 with fragments of more basic lavas. Some of these might obviously 

 have been derived from the rock of the porphyry ridge, but, as at 

 Llyn Padarn, there is a far greater variety of material than can be 

 found in that ridge. Some of the fragments show perfect flow- 

 structure. Professor Bonney has described the microscopic cha- 

 racters of some of these fragments, and has especially remarked upon 

 their glassy character. Among the slides prepared from specimens 

 collected by myself, besides the abundant fragments of felsite (rhyo- 

 lite) there are also numerous pieces of different andesitic lavas and 

 fine tuffs, as well as grains of quartz and felspar, and sometimes a 

 good deal of granular iron-ore. 



That a large proportion of the material of the so-called " Bangor 

 beds " was directly derived from volcanic explosions can hardly be 

 doubted. There appears to have been a prolonged succession of 

 eruptions, varying in intensity, and somewhat also in the nature as 

 well as in the relative fineness of the material discharged. On the 

 one hand, there are coarse massive agglomerates which were pro- 

 bably accumulated not far from the active vents as the result of 

 more violent explosions ; on the other hand, exceedingly fine and 

 well-stratified tuffs which attain a great thickness and serve to 

 indicate the persistence of a phase of eruptivity marked by 

 the discharge of fine volcanic dust. Ordinary sediment was 

 doubtless drifted over the sea-bottom in this district during the 



