Pvof. W. Keejnng — Geology of Aberystwyth. 533 



These several districts form the boundaries which, together with 

 the sea of Cardigan Bay, make up a complete zone enclosing the 

 large area of Mid Wales — an area which is very commonly regarded 

 as a wilderness to the geologist, and a desert to the palaeontologist. 

 Nowhere in this country are fossils abundant, and the rock-beds are 

 in places marvellously contorted. To study this group of Mid 

 Welsh rocks, we could not do better than to make Aberystwyth our 

 head-quarters, and thence traverse the country in various directions. 



First, taking a walk along the shore either to the north or south of 

 the town, the cliffs offer a splendid view of the Aberystwyth group 

 — a great series of bedded rocks in such violent twistings and 

 puckerings as astonish even the most unobservant of people. I 

 know of no better place throughout England or Wales for observing 

 these rock contortions. 



Faults and Joints. — The few dislocations or faults seen along the 

 coast are very small; for example, those just south of Aberystwyth 

 Castle, and again between Aberystwyth and Clarach ; but larger 

 faults, mostly in an east and west direction, are doubtless indicated 

 by the many lead veins to the west. 



The rock structure known as jointing may be well studied in this 

 neighbourhood. A glance at a quarry in any rock will show that 

 the rock material is not all in one solid mass, but is divided up into 

 blocks, sometimes very regularly, sometimes quite irregularly, so 

 that masses may be removed with the wedge and lever without ever 

 breaking across a stone. Such natural planes of division are known 

 as joints. The extent of these joints varies greatly ; at Allt Wen, 

 south of Aberystwyth, some very fine examples are seen cutting 

 vertically and cleanly through tJae rocks, so that in those places 

 where they have been exposed by landslips, which are fi'equent, 

 they stand out like huge walls of well-built masonry. We may see 

 them running thence straight down the cliff and continuing their 

 course across the shore out to sea. 



At the other extreme we have small and frequent jointing, hard 

 to distinguish from cleavage, as is seen in some of the grit-beds near 

 Llanilar, One cannot detect any such regular persistency in the 

 direction of the lines of jointing as we shall afterwards find in the 

 case of cleavage ; on the other hand, there is a more intimate rela- 

 tion between the jointing and the sloping of the rock-beds. Some 

 of the hard sandstones of Plynlimmon are jointed into rude columnar 

 masses. 



Back varieties. — The rocks of Mid Wales are of few kinds, and 

 they are remarkably similar throughout the district. Beds of grit 

 of that characteristic Cambrian type mixed with clayey material 

 known as Grey wacke (originally a muddy sand) , sometimes coarse 

 grits, hard sandstones, and occasionally a pudding-stone or con- 

 glomerate bed (originally pebble beds), are the forms in which the 

 ancient sandy deposits occur, whilst the clay deposits are now found 

 as irregular shale, mudstone rab (a mudstone breaking into small 

 fragments when exposed), wrack (a coarser larger form of ruh), 

 pencil rab (breaking into long fragments — as seen under Allt 



