664i REPORT— 1903. 



Nullipores also enclose sand grains in their stony tissues, but they are spo- 

 radic in their arrangement, and can readily be distinguished from those cemented 

 by polyzoa. 



In some cases alternate growths of nullipores and polyzoa colonies give both 

 types in the same section. 



Some of the blocks of calcrete brought up were of large size, 2 or 3 feet in dia- 

 meter ; and these are but samples of the rocks now being formed which extend in 

 some of the paars over a distance of many miles. 



6. On the Igneous Hocks of the Berwyns. 

 By T. H. Cope and J. Lomas. 



Owing to cross folding a dome-like structure has been impressed on the Ber- 

 wyns. From the axis which lies about Llaurhaiadr-yn-Mocbncint and Craig-y- 

 Glyn the beds dip outwards on every side. The arch of the dome has been 

 denuded, so that we get shales and limestones of Llandeilo age occupying the 

 central area, while slates, grits, and limestone of Bala age foroa an almost con- 

 linuous ring of hills on the margins. 



Igneous rocks are associated with the sedimentaries. Three bands in tlie peri- 

 pheral series can be traced continuously for a distance of thirty miles from the 

 Mountain Limestone beds which overlap the series on the east, through the hills 

 above Corwen and Bala to the Vyrnwy watershed. A fourth band also occurs in 

 this series about Llanarmon. 



In the central area other igneous rocks are exposed, generally of a more acid 

 type. 



The igneous series have been regarded as contemporaneous volcanic ashes, and 

 recorded as such in the Survey maps. We have failed to tind any instance of 

 nndoubted contemporaneous action, and regard all the igneous as intrusive. In 

 places they are seen to cut across tlie sedimentaries at right angles to the strike. 



In this paper we only deal with a small part of the peripheral .series as dis- 

 played about LlansantU'raid-Glyn-Ceiriog where the river Ceiriog in cutting a deep 

 gorge across the strike of tbe beds has exposed magnificent sections. 



Sheet No. 1. — The outermost bed is well seen in tbe quarries at Coed-y-Glyn, 

 on the west side of the valley, and in a small cutting on the hillside on the east 

 side. It is 45 feet thick on the level of the road, but thins out rapidly to the 

 north, as at a short distance away it only measures 28 feet. Baked slates lie in 

 contact on both its upper and lower surfaces 



The rock consists of a felted aggregate of felspar microliths, and is aphanitic in 

 texture. The upper margin for 5 feet and the lower part for 2 feet are amygda- 

 loidal. Near the upper surface tbe microscope reveals ilow-brecciation, broken 

 fragments of the rock lying in a bond of grey translucent chalcedony. 



Sheet No. 2,- — This band, about 165 feet thick, has been quarried extensively on 

 the face of the steep crags overlooking Pandy, at Cae Deicws, and in tbe large 

 quarry opposite Coed-y-Glyn. Indurated slates and grits border the sill on Iwih 

 surfaces, and large masses of slate occur as inclusions. A band of white rock of 

 very varying thickness occupies the middle, which under the microscope shows 

 large idiomorphic quartz and orthoclase felspar crystals in a felsitic ground mass. 

 The margins are intensely sheared, grey in colour, and include a great number of 

 slate and limestone fragments along with angular pieces of the white uncleared 

 central portion. 



Sheet No. 3. — This sheet is well seen in Coed Errwgerrig and can be traced 

 across the bed of tbe river to the east side of the valley at Cwm Glwyd. "While the 

 main mass resembles Sheet No. 2 in composition, it includes fragments of quartz 

 felsite, felsite breccias, and nodular rhyolites arranged in parallel bands. 



Tt is 190 feet thick, and has caused intense metamorphic action on the grits 

 above and slates below. 



Sheet No. 4 is best seen at Ilendre Quarry, where it is worked extensively, and 

 locally known as the Glyn ' Granite.' 



