"Vol. 54.] LIMESTONE OF THE COUNTKY AROUND LLANDTJBNO. 389 



limestone is only 10 feet thick, and it cannot be traced farther west. 

 Lenticular beds of dolomite occur in the limestone south-west of 

 St. Tudno's Church, and in the steep cliffs above Gogarth, while 

 south-west of the Head there are beds and dykes of dolomite along 

 the side of the road ; but chert is rare in the subdivision, if it be 

 found at all. 



About 20 feet from the top of the Middle White Limestone there 

 are some beds of sandstone exposed in the large quarry west of 

 Roft Each. The sandstone is of irregular thickness, varying from 

 10 to 20 feet ; it changes from a soft sandstone into a very hard 

 arenaceous limestone, and contains occasional small quartz-pebbles. 

 In colour it varies from white to red, though principally of a grey 

 shade. This rock is on about the same horizon as the sandstone- 

 conglomerate in Anglesey. 



Although the base of the Middle Limestone is well defined by the 

 bed of shale already described, and rests on massive beds of dolomite 

 representing the Lower Brown Limestone in the Happy Valley, it 

 is difficult to trace it farther west than the Old Road, in con- 

 sequence of the conversion of its lower half into dolomite, as shown 

 in the map (fig. 2, p. 386). The upper limit of the change is 

 well seen in a section along the south side of a short road from 

 Roft Each. It exposes dolomite with limestone over it, string- 

 courses of dolomite with limestone on the north and dolomite on the 

 south side, and in another place a cluster of such strings of dolomite 

 including fragments of unaltered limestone. The section seems to 

 afford positive proof that the whole of the rock was originally 

 limestone, and that long after its deposition it was converted into 

 dolomite. It is also remarkable that the dark and more impure 

 limestone was more readily changed than the white limestone, for 

 the alteration is limited for the most part to the Lower Brown 

 Limestone, the dark beds at the base of the Middle White, and to 

 portions of the Upper Grey Limestone. At Pen Morfa Lodge the 

 cliffs present lofty masses of dolomite, while farther along the 

 Marine Drive beds of limestone are interstratified with it, and 

 finally the dolomite ceases to occur. 



The lodes and faults shown on the map are not distinguished 

 from each other, for several of them are dislocations of the strata 

 that have been worked for ore. The four parallel lodes may be 

 faults, but there is no proof of such a conclusion : it seems pro- 

 bable that they were fissures which originated with the bending of 

 the strata, and that they formed channels for conveying water 

 containing magnesium carbonate, which altered the limestone along 

 their boundary-walls, sometimes for a considerable distance. Pearl- 

 spar, copper-pyrites, and other minerals were afterwards deposited 

 in the open fissures, principally in the dolomitized portions and 

 about the intersections of the veins. From an examination of 

 specimens collected from the old mine-heaps it seems that the 

 crystals of pearlspar were always deposited first, then usually the 

 chalcopyrite, and finally the calcite. The anthracite was probably 

 introduced as a liquid bitumen, which afterwards lost its volatile 

 hydrocarbon constituents. 



