Murchison's Silurian System. 537 



In the descriptions which follow, it will be seen, says Mr. 

 Murchison, that the distinctions implied by these names are 

 not absolutely peculiar to any one of the three divisions. 

 Thus fossil beds occur partially both at the bottom of the 

 first, and in the middle of the second division, while con- 

 glomerates sometimes take the place of tilestone in the 

 lowest strata. 



Mr. Murchison's descriptions are so connected with loca- 

 lities, that it becomes a very difficult matter to confine our 

 abstract to the rocks alone, without reference to places, and 

 still to preserve the true meaning of the author ; difficult 

 as this is, we must endeavour to do so, as it would be too 

 much to attempt to quote the whole of the valuable details 

 which Mr. Murchison has brought together. The southern 

 face of the Carmarthen and Brecon mountains, like most 

 other parts of the edge of the South Welsh coal basin, 

 slopes in an inclination of from ten to twelve degrees to- 

 wards the centre of the coal-field, and is usually covered 

 with turf bogs, beneath the bog a conglomerate of white 

 quartzose pebbles in a red matrix forms the upper member 

 of the old red sandstone. 



This stratum is seen dipping beneath the carboniferous 

 limestone, where it often assumes a slightly calcareous quali- 

 ty in the cement, with pink and white nodules of quartz, 

 from the size of mustard seed to two or three inches, some- 

 times mixed up with grains of a green compact felspar ; other 

 and lower beds are however pure quartzose conglomerates. 

 " When fresh quarried the conglomerate is sometimes of a 

 pink or reddish colour, but after long exposure it frequently 

 becomes nearly white, in which state it might be mistaken 

 for the coarser beds of millstone grit, but it is separated 

 from them by the carboniferous limestone. The conglome- 

 rate beds occupy a thickness of about 200 feet, and pass 

 down into chocolate-brown, and reddish coarse grained 

 sandstone, with blotches of red shale, and occasionally a 



