38 GEOLOGICAL HISTORY. [CH. 



volcanic ashes and lavas. Much of the typical Welsh scenery 

 owes its character to the folded and weathered rocks laid down 

 on the floor of the Ordovician sea, on which from many centres 

 of volcanic activity lava streams ^ and showers of ash were 

 spread out between sheets of marine sediment. The Arenig- 

 Hills, Snowdonia, and many other parts of North and South 

 Wales, parts of Shropshire, Scotland, Sweden, Russia, Bohemia, 

 North America and other regions consist of great thicknesses 

 of Ordovician strata. 



IV. Silurian. 



Passing up a stage higher in the geologic series, we 

 have a succession of conglomerates, sandstones, shales, and 

 limestones; in other words, a series of beds which represent 

 pebbly shore deposits, the sands and muds of deeper water, 

 and the accumulated debris of calcareous skeletons of animals 

 which lived in the clear water of the Silurian sea. The term 

 Silurian (Siluria was the country of Caractacus and the old 

 Britons known as Silures^) was first applied by Murchison in 

 1835 to a more comprehensive series of rocks than are now 

 included in the Silurian system. The rocks of this period occur 

 in Wales, Shropshire, parts of Scotland, Ireland, Scandinavia, 

 Russia, the United States and other countries. After the 

 accumulation of the thick Ordovician sediments, the sea-floor 

 was upraised and in places converted into ridges or islands of 

 land, of which the detritus formed part of the material of 

 Silurian deposits. The limestones of the Wenlock ridge have 

 yielded an abundant fauna, consisting of corals, crinoids, 

 molluscs and other invertebrates. In this period we have 

 the first representatives of the Vertebrata, discovered in the 

 rocks of Ludlow. In fact, in the Silurian period, "all the 

 great divisions of the Animal Kingdom were already repre- 

 sented'^." 



1 Murchison (72), p. 5. ^ Kayser and Lake (95), p. 88. 



