366 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 3, 



As in numerous instances of dykes of igneous rocks, even of mo- 

 derate dimensions, we see that they have so acted on the rocks they 

 traverse as to change a soft sandstone into a compact flinty slate, and 

 an earthy hmestone into a crystalhne marble, so we might have ex- 

 pected that a mass nearly half a mile in width would certainly have 

 produced similar effects ; but I did not discover any signs of meta- 

 morphic action, unless some slight induration of the slate in one or 

 two places may be so considered. 



I would particularly call the attention of geologists who may here- 

 after visit this spot to the phsenomenon of the laminated structure or 

 slaty cleavage of some of the smaller dykes in the cliff, and especially 

 that indicated by the letter c in the above description, in which the 

 lamination is at right angles to the sides of the dyke, and the stone 

 is scarcely to be distinguished from a quartzose arenaceous rock. 



. I would also call attention to the existence in the great porphyry 

 dyke of greyish-green spots and stripes, similar to those so commonly 

 met with in red sandstones, arising probably from the iron in both 

 parts being in different states of oxidation. The change from red to 

 the grey or greenish tint has been ascribed to the presence of decom- 

 posed vegetable matter*; but it is evident that such an explanation 

 will not apply in the case of an igneous rock. 



2. On the May-Hill Sandstone, and the Paleozoic System of 

 the British Isles. By the Rev. Professor Sedgwick, F.R.S., 

 F.G.S., &c. 



[Abstract.] 



In a former communication t the author and Professor M'Coy had 

 shown that the sandstone of May-Hill contains a group of fossils of 

 the true Wenlock type, and that it must therefore be cut off from 

 the Caradoc sandstone, and arranged with the base of the Wenlock 

 group ; also that on the western flank of the Malverns, the upper 

 portion of what had been called Caradoc sandstone is the equivalent 

 of the May-Hill sandstone. In the present paper Professor Sedgwick 

 treated of the Mathyrafal and Glyn Ceiriog sections, on the east side 

 of the Berwyns, and sections on the western side, together with the 

 sections of the Pentamerus limestone of Norbury and Linley, on the 

 flanks of the Longmynd ; the sections of Horderley, the Ouny, and 

 of Caer Caradoc ; and lastly, the sections of Builth, Llandovery, and 

 Llandeilo. 



From the examination and consideration of these sections, and of 

 their fossil evidences, the author concludes that in those great phy- 

 sical regions there is not so much as one continuous unbroken section 

 throughout which we can ascend in the way of passage from the 



* " On the Colouring-matter of Red Sandstones and of Greyish-white Beds 

 associated with them," by John William Dawson. — Quarterly Journal of the Geo- 

 logical Society, vol. v. p. 25. 



t Quart. Journ. Geol. Soc. vol. ix. p. 215. 



