234 



THE GEOLOGY OP THE DISTRICT. 



By the Rev. W. S. SYMONDS, President of the Malvern Club. 



On the present occasion our time is so limited, owing to the unpro- 

 pitious hour of the returning train, that I can do little more than give a very 

 humed and imperfect sketch of the interesting geology of the Builth district. 

 You are probably aware that during the far distant geological period, known 

 as the Lower Silurian epoch, there were active volcanoes, and two periods 

 of volcanic outburst in what is now North Wales. Volcanic action seems to 

 have commenced in the Cambrian period, and to have increased during that 

 of the deposition of the Lingula Flags, but the internal forces became more 

 intensified during the period when the strata known as the Llandeilo deposits 

 were accumulating in the Lower Silurian seas. The second period of volcanic 

 action occurred towards the close of the Caradoc or Bala period, and the noble 

 Snowdonian hiUs, which run from Moel Hebog by Carnedd Llewelyn to 

 Conway, are composed of strata of the Caradoc age, interstratified with sub- 

 marine lavas and volcanic ashes, all of which have long since been upheaved 

 into the noble hills the geologist loves to traverse, and which display along 

 their scarped and rugged rocks the histories he learns to read. 



It was during the earlier period of this volcanic activity that the ancient 

 igneous rocks of BuUth were erupted. The great Arenig mountains in 

 Merionethshire are made up of repeated streams of lava interstratified with 

 sedimentary strata of the Llandeilo age, containing fossils and volcanic ashes, 

 and these Arenig rocks are older than those of Snowdon, as proved by the 

 superposition of the Snowdon rocks and their organic remains. The Arenig 

 rouks belong to the LlandeUo period, which preceded the Caradoc period. 



It was during the latter part of the Llandeilo period (upper LlandeUo), 

 and when volcanic action was also rife in North Wales, that the volcanic 

 forces burst forth through the upper Llandeilo strata in this region of 

 BuUth. 



Here you have repeated on a smaU scale the interesting geology of some 

 of the grandest hills in North Wales. Here you may see old lava streams 

 of the Llandeilo period, and the interstratified beds of volcanic ash which 

 were showered down from a neighbouring crater into the sui rounding sea, and 

 which aah and pumice sunk graduaUy through the waves, and became inter- 

 bedded with strata, which contain the fossU remains of the mollusca and 

 crustaceans which inhabited the LlandeUo seas. I last month saw strata of 

 the Tertiary fresh water lakes of Auvergne, intei bedded with stratified ashes 

 and peperino, and traversed by lava dykes, whUe the stratified deposits were 

 full of the cases of caddis worms, and fresh water shells that inhabited the 



