160 E. Greenhj — Age of Later Anglesey Dyhes. 



III. — On the Age of the Later Dykes of Anglesey. 



By Edward Greenly, F.G.S. 



rriHE existence in Anglesey of a series of dykes later than most of 

 _L the rocks of the island has long been known. Some of the 

 more conspicuous ones were described by PTenslow, many more 

 appear on the map of the Geological Survey, and petrological 

 descriptions were given some years ago (Geol. Mag., 1887-88} b}^ 

 Mr. Marker. 



They have been Wtherto regarded as of Post-Carboniferous but 

 Pre-Permian age, on the ground that, while traversing the Coal- 

 measures of tlie Malldraeth Marsh, one of them has been observed 

 to terminate at the base of the overlying red strata. Sir A. C. 

 Ramsay distinctly states that this evidence is derived entirely from 

 the old coal-workings, and that at the time he wrote the reel beds, 

 regarded by him as Permian, were nowhere visible at the surface. 

 ("Geol. N. Wales," 2nd ed., p. 266.) 



Now, however, some of the red beds in question ai'e exposed in 

 the bottoms of the railway cuttings on either side of Holland Arms 

 Station. The sections are shallow, the cuttings being grassed over 

 to within a few feet of the bottom, but are tolerably clear where 

 visible. The rocks seen are variable red sandstones and marls, often 

 green-mottled with occasional thin beds of harder j^ellow and white 

 sandstone and grey shale. While some of the types are such as 

 elsewhere occur in the true Coal-measures, others are of well-known 

 ' New Red ' aspect, and might, as far as lithological characters go, 

 be even of Triassic age.^ T'hej'^ closely resemble the rocks seen on 

 the old pit spoil-banks, among which I have found red sandstones 

 with secondary outgrowths of quartz showing crystal facets, like the 

 well-known rocks of Penrith and other localities. There are beds, 

 moreover, containing fragments of chert in a dolomitic matrix, and 

 some with rolled fragments of limestone ; and due weight must still 

 be attached to the statement that certain faults fail to traverse these 

 red beds in the old coal-mines. 



It appears, then, that while the question of the age of these rocks 

 cannot be regarded as settled, there is no fresh evidence to upset, 

 and some to support, the view of Sir A. C. Ramsay that they are of 

 Post-Carboniferous age. 



Now, in the cutting south-west of the station, these beds are seen 

 to be traversed by two dolerite dykes, one on each side of the 

 Holyhead Road, that on the north-east being about 50 feet and 

 that on the south-west side about 110 feet in width. Their intrusive 

 relations are clear, and one of the grey shales shows small spots 

 and is perceptibly indurated near the dyke. They break up 

 spheroidally, and although the bulk of the material of both dykes 

 is quite decomposed, yet the spheroidal cores are not only hard 

 but remarkably fresh and well preserved. The dyke on the 



^ Some of my former colleagues of the Geological Survey, who have lately had 

 much experience of the Eed Rocks of South Wales and the Midlands, kindly 

 examined some siiecimeus for me. 



