MONIAN SYSTEM OP EOCKS. 



513 



there is one long range of brown limestone and another of smaller 

 size, but their relations to the other rocks are not seen. I have 

 found some also on Llanddwyn Island, which shows too the agglo- 

 merates, diabases, and slates. The constant association of these 

 limestones with rocks which we have independent reasons to believe 

 are of volcanic origin, or show great local disturbance, indicates a 

 relationship between them, just as we find calcareous springs most 

 frequent and powerful in modern volcanic districts. 



Such are the principal phenomena of this wonderful region, which 

 is well worthy of a visit by any one interested in the ways of rocks 

 whether of Pre-Cambrian or of modern date. Its nearest point is 

 situated about four miles from Bodorgan Station. 



Summary of the Eastern District, 



The governing feature of this district is the existence of nume- 

 rous masses of foliated diorite of peculiar constitution, which passes 

 into the ordinary form near Holland Arms, and is proved to be of 

 igneous origin by its identity with an intrusive rock seen in LlangafFo 

 cutting. The main rocks into which this diorite intrudes are 

 grey gneiss and mica-schist, the former rather to the west, and the 

 latter, as its disturbed representative, in the east, and both rising 

 into considerable heights in Mynydd Llwyddiart. The result of 

 the intrusion is sometimes to indurate and give a felsitic aspect to 

 the gneiss, and sometimes to produce contortions with a greater 

 development of mica and chlorite. Towards the north-west these 

 gneisses pass first into irregular chloritic rocks, comparable to those 

 on the mainland in the western district, and then into a series of 

 irregular ashes and marbled slates, which have the usual accompani- 

 ments of sporadic limestones and quartz-knobs, with occasional 

 diabase-flows. On the western side, at either end, are found slices 

 belonging really to the central district, being intermediate in vertical 

 position between this and the eastern. In the north it commences 

 with chloritic ashes and ends with the slates and limestones, 

 showing the same association as in the north-east. In the south 

 there is an extraordinary development of rocks, which behave in a 

 manner only to be matched in the heart of modern volcanic 

 districts. Different parts of the series are overlain by the basement 

 Cambrian on the east. 



The Noethern Disteict. 



"We are now in a position to examine the Northern District. 

 Its description would perhaps have come more naturally between 

 those of the Western and Central ; but, in face of the doubts which 

 its want of metamorphism has produced, we can study it with more 

 confidence when justified by the results obtained in other areas. 

 We have, on the one hand, Dr. Callaway referring the whole to his 

 slaty series, and, on the other, Prof. Hughes suggesting that the rocks 

 succeed the black slates of the Ordovician in time, as they do in 

 space, and are consequently no older than the Bala group. 



2m2 



