572 MR. H. WOODS ON THE IGNEOUS ROCKS [NOV. 1 894, 



andesite on the east of the Llanelwedd diabase-porphyrite than 

 that on the west. The specific gravity of one of the pebbles was 

 found to be 2-58, and the silica-percentage 51*6. 



Fossils are not very common in the ash, but I have found several 

 specimens of Ortliis calligramma, Dalm. M'Coy l also records the 

 following, which are preserved in the Woodwardian Museum : — 

 Crania divaricate (M'Coy), Leptama sericea, Sow., and Serpulites 

 dispar, Salt. 2 In the third quarry south of Tan-y-graig I also 

 found specimens of what appear to be polyzoa. Dr. Gregory kindly 

 examined these, and he says that they seem to be allied to the 

 genus Drymotrypa. These species are not sufficient to fix the 

 horizon of the ash, but as it is overlain by Llandeilo Shales con- 

 taining Ogygia huclii, etc., it is probably of Lower Llandeilo age. 



Fig. 1. — Section immediately south of Tan-y-graig. 



E 



too Feet above Sea-level 



[Scale : 1 inch = 880 feet.] 



a = Andesitic ash. 



b = Andesite. 



c — Diabase-porphyrite. 



d = Diabase. 



e = Llandeilo Shales. 



On the Geological Survey map the ash just described is shown as 

 greenstone, but this appears to have been a slip in colouring, since 

 its nature is correctly indicated on the Horizontal Section (Sheet 5, 

 No. 1), where it is spoken of as ' Trappean Conglomerate ' in the 

 first edition, and ' Felspathic Tuff and Conglomerate ' in the second. 



In the lowest part of the quarry at Tan-y-graig, underneath the 

 andesitic ash, is seen a greyish rock with numerous white felspars, 

 resembling closely, in hand-specimens, the diabase-porphyrite already 

 described ; under the microscope, however, it is seen to be quite 

 different from that rock. It is composed of a fine-grained base con- 

 taining plagioclase-felspars of various sizes and usually of irregular 

 outline ; some calcite and chlorite are also present. Although 

 my sections are not absolutely conclusive, I think there can be 

 little doubt that the rock is an ash : its silica-perceutage is 61*44. 



1 'British Pah-eozoic Fossils' (1852), p. 373. 



2 The Rev. T. C. Davies, formerly of Builth, informs me that many years ago 

 he found in the ash at Tan-y-graig a specimen of Asaphus tyrannus, and believes 

 that he gave it to Dr. Grindrod. No reliance, however, can he placed on 

 the determination of the species. On Horizontal Section No. 1 of Sheet 5 

 (Geol. Survey) there is written under the ash at Tan-y-graig ' Asaphus laticos- 

 tafus,' but Mr. E. T. Newton, F.R.S., informs me that tlie specimen is not in 

 the Museum of Practical Geology, and he thinks it must have come from the 

 Llandeilo Shales, not from the ash, since Salter, in his Monograph on the 

 Trilobites (p. 158), says that the only specimen known is a tail from the Llan- 

 deilo Flags of Maen Goran, near Builth. 



