LESSONS IN GEOLOGY. 



that this period WM the dawn of life. 



othrrwiBO. 



may i.uvr 



THE SILURIAN HT8TKM. 



We now enter npon the true sedimentary utrata. The 



in. T|. in.- urtion hat) not penetrated to this system, and the bed* 



ar preserved to OH in the condition in which they were deposited, 



it they have been tilted and upheaved by igneous agency 



from U'lifiith. The term Silurian was selected by Sir li. Murohi- 



son because the system is typically exhibited in North Wale*, 



throughout the territory anciently occupied by the tribe of the 



They are moat favourably placed for examination, 



:i>y have been " set on edge," BO that aa you walk along 



a liiif in an easterly or westerly direction, you pats over the 



exposed edges of the successive strata. The different series of 



the system and their sub-divisions are as follow : 



SILURIAN 



I. I. 



1, Downtown 

 sandstones ; 2, A y- 



limestones ; 

 3, Lower Ludlow 

 shales. 



II. Wenlock Series. 

 1, Wenlock lime- 

 stone ; 2, Wenlock 

 .-hale; 3, Woolhope 

 limestone; 4,Upper 

 Llandovory grits 

 and shales. 



LOWER SILURIAN 

 ROCKS. 



III. Llandeilo 

 Series. 1, Lower 

 Llandovery sand- 

 stones and slates ; 

 2, Caradoc sand- 

 stones ; 3, Bala 

 beds ; 4, Llandeilo 

 and Lingula flags ; 

 5, "Bottom rocks." 



LOWER SILURIAN 

 ROCKS. 



Sir Charles Lyell 

 adds a third divi- 

 sion to the Silurian 

 strata, which ho 

 terms the Middle 

 Siluria, and in it 

 he places the upper 

 and lower Llando- 

 very series, which 

 consists of the Tap 

 annon shales, the 

 MayHill sandstone, 



Peutamerns limestone, and the Llandovery slates, the aggregate 

 measuring about 2,600 feet in thickness ; but, as a very definite 

 lino of division is wanting, we have divided the middle Silurian 

 between the upper and lower. 



The Llandeilo series is so named from a town in Carmarthen- 

 shire, about which the rocks are exhibited, where they reach a 

 thickness of nearly 1,500 feet. Here wo moot plentifully with 

 fossils ; in such abundance, indeed, do somo species occur as 

 the graptolites that some geologists ascribe the dark colour of 

 the slates which lie on the top of the series to tho carbon 

 derived from these animals. 



But tho most characteristic fossil of the Silurian period is the 

 irilobite (Fig. 30), a crustacean with which those early M ;n 

 must have swarmed. They evidently had a jointed body, and 

 probably could spring like our shrimps. There are many va- 

 rieties, each bed of the system possessing some, if not peculiar to 

 itself, yet which predominates in that locality. However much 

 they may differ in other respects, they have all one feature in 

 common they are all three-lobed, hence their name ; that is, 

 their body is divided into three lobes by the penetration of a 

 ridge down the centre ; this is very evident from the figures. 

 The trilobito possessed a wonderful eye; indeed, it seems a 



pyramid of eyes, so that it could enrvey the waters oa all side* 

 of it, above as well as around. This higUy-deveJoped eye is an 

 argument against Darwin's theory of development, for hi the 

 very earliest of the fossaliferooa strata we shoold earely tad 

 very crude organs, the unoooth fore-elders of those highly- 

 organwod senses which the animal* of oar epoch of life poetess; 

 and yet it is not so, for one of the very esrUest area tares pos- 

 sesses as fine an eye as any which has soeceeded it, a very plain 

 indication of the power of a Creator. The trilobite, which ia 

 found chiefly in the T.landeilo rocks, is the Atapkut lyraaaw 

 (Fig. 31), and is therefore considered characteristic of these 

 beds. The Ogygia. Duehii is also a feature of the period. The 

 trilobites have been closely examined by If . Barrande, who dfa 

 covered that they underwent metamorphoses as oar present 

 He traced them, from their eaeape from the egg, 

 through all their 

 '} ii/-t tint.! '.}.< T 

 arrived at the adult 



crustaceans. 



'* , 



changes an many, 

 it is not to be won- 

 dered at that f re- 



i. :.'. 



je.de in 

 a fossil which fe 

 only a young trflo- 

 lite to be something 



The OrmphMm 

 are Hydroaoa, and 



different forms, 

 some of which are 

 represented in oar 

 illustration. The 

 didymograpsns 

 (Figa. 32 and 32a> 



- 



(Figs. 33 and 33a) 

 do not appear 

 above the lower 

 Silaria. In Fig. 

 34 the didymo- 

 grapsns is enlarged. 

 Arms are extruded 

 oat of the lateral 

 tubes, which were 

 joined to the main 

 trunk of the grap- 

 tolite, which was 

 in the long tobe 

 ont of which 

 the short arms 

 branch. 



In Fig. 35 is re- 

 presented another 



of the hydrozoa of the lower Silnria the Rattritc* Mvyrimii, 

 having spines projecting from it like the teeth of a rake. 



Besting upon the Llandeilo series are the Oarodoe Mftd- 

 siones and Bala beds. These, net including the trap rocks 

 which are interstratified with them, attain the thickness of 

 <J,000 feet. Caer Caradoc, where the sandstone shows itself, is a 

 mountain in Shropshire. The rook is very shelly, and ooataias) 

 many fossils. The IVinvefev* Caraeiaci is found thl oaf anal 

 this sandstono ; it is a beautiful trilobite, which has six rings ia 

 the thorax, tho three bosses very prominent, and two long con- 

 tinuations of the head, which will be figured ia some of the 

 upper Silurian trilobites. The Stivphomena yramdu (Fig. 86) 

 and the Orthu vespcriilio (Fig. 37) are also characteristic fossils 

 of the Caradoo beds. At Bala, in Merionethshire, a lisnettrmo 

 of this age occurs very foaailiferoas. In it the trilobite life 

 reaches its maximum. It yields fossil star-fish, and the HtMae. 

 p/urtfea ooUtciu (Fig. 38) and others of the Cyttida lately 

 classed with the Radiala. The mouth of the creature was at the 

 top, and it was fixed to the rock by a stem which was attached 1 

 at the opposite side to the month ; bat this stem is never foaui 

 still adhering. The Bala limestone is about 30 feet thick 

 Lower Llaiw2oi*ry Btdi are grey and brown grita and VM- 



