82 THE STORY OF THE EARTH. 



the Denbighshire grits, Wenlock and Ludlow beds, 

 and the Downton sandstone series, are combined 



>rm the Silurian group. 



Prom the physical history of the deposits, 

 the: mnd for dividing the rocks in this 



way, but from consideration of the life they 

 contain, the whole might well be combined, and 



iped together as ten successive series, with 



ten distinct fauna, which more or less resemble 

 the life of similar natural history provic 

 superimposed on each other, and preserved suc- 



SSively in sediments in the same area. 

 At first the old rocks which comprise the 

 Longmynd groups, and the Harlech and Llan- 



beris -late-, which rise 1600 feet above the sea, 

 in the Longmynd Hills in Shropshire, consist of 

 slates, sandstones, grits, and conglomerates; with 

 very few fo>>ils. The water-worn pebbles in them 

 prove deposition under ancient shore conditions; 

 and they are associated with beds which show the 

 ripples <>f waves, runnels of rills on the shore, in- 

 terlacing cracks formed by the heat of the sun, 

 prints of raindrops, and burrows of sea-worms 

 allied to the living Arenieola. lew fossils 



have found in the Longmynd. They are 



. more numerous m the Bangor country of 



'lire. There the rocks are represented 



by green and purph . which stretch from the 



banks of the < tgwen through the lake of I.lanlu 

 and the I'enrhyn slate quarries. In South \\ | 



in the section neai Si 1 David's, the int< 



test, rhe ' and 



ind there-, with red, purple, and 



ippear to be on the same | 

 • e Bel ■ - I Llai 



e quarries. I i ■■■ ards the base of this group 



