THE GEOLOGICAL RECORD 199 



and the cherts bearing radiolaria, give evidence of excessively 

 slow deposition in quiet deep oceans, a conclusion borne out by 

 the wide distribution of floating forms of life like the graptolites. 

 In addition to these there is a great advance in numbers and 

 organisation of the trilobites, abundance of spire-bearing and 

 horny brachiopods, and increasing development of cephalopods. 

 But the steadfast advance in the graptolites gives at once an 

 impressive picture of the evolution of the group and an admirable 

 key to the age of the deposits. Ores of lead, zinc, and barium, 

 occur in the veins of the rocks, which also yield supplies of building 

 stone, road metal, and roofing slate. The beautiful landscapes 

 of the Lake district (Fig. 2), North Wales (Fig. 68), and Shrop- 

 shire, are due to the presence of bands and masses of igneous rocks 

 associated with the softer sediments, and the contrasting scenery 

 of the Southern Uplands of Scotland is the result of the minor 

 importance or absence of igneous rocks there. 



The Silurian System is made up either of alternations of shales 

 and limestones or of slates and grits. Its landscape in the former 

 case consists of edges (or wolds) and vales ; in the latter of pastoral 

 uplands or rugged moorlands. The rocks were laid down either 

 in continuous areas of shallow water with shifting currents, or in 

 calmer areas in which calcareous organisms furnished supplies of 

 shells and tests for limestones, occasionally interrupted by in- 

 cursions of fine sediment . Volcanic activity was in abeyance . The 

 life includes trilobites (Figs. 76 and 26) and graptolites, the former 

 reaching a high stage of perfection, the latter dying out at the 

 end of the period. Corals are abundant, forming limestone reefs ; 

 indeed, the period might almost be spoken of as that of corals 

 and crinoids (Fig. 24). Brachiopods are very numerous, especi- 

 ally Pentamerus, cephalopoda important, polyzoa abundant, and 

 there are numbers of lamellibranchs and gastropods. The earliest 

 fishes appeared at the end of the Period, cartilaginous and 

 armoured for the most part ; and about the same horizon have 

 been found the earliest land plants (cryptogams), and a giant 

 group of the merostomata, the eurypterids. Few Formations 

 yield more fossils or better preserved. The limestones have been 

 extensively quarried for lime-burning and for smelting. 



The rocks of the Devonian System in Devon and Cornwall are 



