CHAP. IV.] SILTTBIAN PERIOD. 41 



these being respectively, in upward succession, the Wool- 

 hope, the Wenlock, and the Aymestry limestones, but at 

 Tortworth, May Hill, and Malvern, there is a fourth lime- 

 stone between the Wenlock and the representative of the 

 Aymestry, which might be regarded as a distinct horizon. 

 This great shale and limestone series is from 2,000 to 3,000 

 feet thick. 



The highest Silurian beds in this region consist of yellow 

 sandstones and reddish shales, the latter passing up into red 

 laminated micaceous sandstones and marls, some of which 

 contain marine fossils; these beds form a passage from 

 Silurian to Old Eed Sandstone, and were evidently formed 

 in shallower water than the dark mudstones and lime- 

 stones of the Wenlock series. 



As regards the eastward extension of the Silurian rocks 

 very little is yet known ; the most easterly outcrop is near 

 Walsall and Barr in South Staffordshire, but Wenlock 

 shales have been found below the cretaceous rocks in a 

 deep boring at Ware, near Hertford, so it is probable that 

 they have a wide subterranean extension beneath the south 

 and east of England. 



The subdivisions which have been established in the 

 limited area where the Silurian rocks were first described, 

 will not apply to the rocks of the same age outside that 

 area. When traced in any westward direction, a great 

 change takes place in the lithological character of the 

 strata : all the limestones thin out and disappear, and the 

 soft mudstones change into a series of hard shales, flags, 

 and grits. In central and northern Wales these alter- 

 nations of shales and sandstones attain a thickness of 

 from 5,000 to 7,000 feet, and this change suggests the 

 existence of a considerable mass of land to the westward. 



The Silurian rocks of the Lake District are similar to 

 those of North Wales they have a conglomerate at the 

 base which lies unconformably upon Ordovician rocks, and 



