256 The Rev. A. Sedgwick's Outline of 



species of the Upper Silurian fossils of Westmoreland, that nine- 

 tenths of them agree specifically with those described in the ' Silu- 

 rian System,' and that their arrangement in the actual sections ad- 

 mits of no rigid comparison with the subordinate groups of the system. 



§ 4. Other formations over the preceding. — Successive dislocations 

 of the Welsh system, $c. 



After briefly noticing one or two masses of red sandstone and 

 conglomerate (referred to the old red sandstone), the author de- 

 scribes the extraordinary position of the mountain limestone in the 

 Vale of Clwyd, and infers that it underwent its greatest dislocation 

 before the period of the new red sandstone, which is found appa- 

 rently in a great bay of the dislocated limestone. He then gives, in 

 the following order, a synopsis of the principal movements which have 

 affected the different formations within the limits described in the 

 paper. 



1 . The oldest movements of which we have any distinct trace, 

 were those which gave the north-eastern strike and threw the moun- 

 tain masses into undulations. 



The highest points of the Carnarvon chain (measured along the 

 strike from Conway) are upon an arch, the crown of which is the top 

 of Snowdon ; and the south end of the arch is broken off by the 

 Tremadoc fault. At what time this position was effected does not 

 appear ; but the tension by which it was produced broke the back of 

 the chain in three places, producing the rudiments of the three great 

 passes across the Carnarvon chain. 



2. Afterwards, a series of movements gave the W.N.W. impress 

 both to the older system at the north end of the Berwyns, and to 

 the upper system in Denbighshire. The author considers that the 

 extraordinary confusion in the position of the beds in some parts of 

 the Berwyn chain is caused by the intersection of the two lines 

 of principal elevation, viz. the old movement to the N.E. or N.N.E., 

 and the subsequent movement to the W.N.W. Probably after this 

 period were formed the conglomerates at the base of the mountain 

 limestone of Denbighshire. 



3. At a later period was formed the great trough of the Vale of 

 Clwyd. About the same time (and probably before the period of 

 the new red sandstone) was formed a line of great dislocation, 

 marked by a patch of mountain limestone near Corwen, affecting 

 the dips of all the intermediate country as far as the great mineral 

 veins of Minera ; and lastly, bringing up a great mass of mountain 

 limestone near Caergwrle in Flintshire. 



4. The great break of the Menai Straits appears to have taken 

 place after the new red sandstone : and probably about the same 

 time was formed the fault, ranging up the lower part of the Conway. 



5. Lastly, all the external inequalities of the country must have 

 been altered, again and again, since the successive periods of im- 

 press, produced by the above lines of great movement. The whole 

 country has, at very recent geological periods, undergone great 

 changes of level. And the author confidently affirms, that the 

 mountain streams of North Wales, as far as he has examined them, 



