Geological Society. 229 



been cleared away from the abrupt faces and points of the intrusive 

 rock. Bands of imperfect serpentine are frequent between the trap 

 and the limestone. The latter, near the contact, is wholly unstratified, 

 crystalline, and hard, and only resumes its ordinary appearance at 

 a certain distance from the trap. The shale is altered into a hard 

 slaty substance. Coatings and nests of anthracite, together with 

 minute veins of copper ore and iron pyrites, appear near the junc- 

 tion, sometimes running from the trap into the altered deposits. 



The author states that the phenomena are very analogous to 

 those of the Val di Fassa in the Tyrol. 



On the north-eastern prolongation of this line of eruption is the 

 limestone of Nash Scar, which, although at its extremities (Wood- 

 side and Corten,) is demonstrably the equivalent of the Wenlock and 

 Dudley rocks, yet in this central part is a craggy mass of altered 

 and crystalline limestone, the changes in which are doubtless due 

 to the igneous influence which has shown itself on the same line 

 at Old Radnor. 



Llandegley, Llandrindod , and Builth Group. — This large trap- 

 pean district having a length of nearly ten miles and a breadth of 

 five, is very similar in structure and physical features to that of 

 Shelve and Corndon in Shropshire, consisting of sharp ridges of trap 

 and deep trenched valleys in grauwacke, all running from north- 

 east to south-west. It is further analogous in presenting some evi- 

 dence of volcanic eruptions of date contemporaneous with the se- 

 dimentary deposits containing the Asaphus Buchii. Transverse 

 sections near Gelli and Buries, illustrate these phaenomena of re- 

 peated conformable alternation of concretionary felspar and other 

 rocks of igneous origin with stratified shelly deposits. 



The most prominent elevations of trap consist of greenstones of 

 many varieties, felspar rock with quartz crystals, porphyries, both 

 amorphous and slaty, passing into porphyritic greenstone, amygda- 

 loids of various characters, concretionary rocks, claystone, &c. 



The author points out, in some detail, the reckless folly which has 

 led people in this district to seek for coal by driving horizontal gal- 

 leries through the black schist on the sides of these eruptive ridges, 

 endeavours which they have been led to persevere in from the oc- 

 casional presence of small pieces of anthracite near the junctions. 



The altered strata of grauwacke within and around the exterior 

 of these hills are of too frequent occurrence even to be named in 

 an abstract. Among a great number of cases, the author calls par- 

 ticular attention to the south-western termination of the great moun- 

 tain of Carneddau near Builth, where certain lower ridges of green- 

 scone, &c, cut through the shale and calcareous flag containing the 

 Asaphus Buchii, the beds of which are distorted, broken, indurated 

 and silicified, and in some instances changed to a milk white colour, 

 and to a brittle condition resembling some sorts of porcelain. Nu- 

 merous and large crystals of iron pyrites occur in these altered 

 beds, and as the mineral waters of the Park wells issue from them, 

 their origin is supposed to be due to the decomposition of the cry- 

 stallized pyrites. Hence the author infers that the mineral sources 



