Oct. 1, 1S70.J 



HARDWICKE'S SCIENCE-GOSSIP. 



219 



believe there was a much wider extension of sea 

 then than there is even now, when it extends over 

 more than two-thirds of the earth's surface ; and, 

 owing to there having been fewer disturbances at 

 that time than there have accumulated in subse- 

 quent periods, the sea was more equable in depth ; 

 whilst, at the same time, the dry land was less dis- 

 tinguished for mountain-chains. In consequence 

 of the equable depth (or nearly so) of the sea, and 

 of the similar climature which the entire surface of 

 the world enjoyed alike, there was less difference 

 in the animals and plants of various geographical 

 zones ; but this principle was in existence, although 

 nothing like so broadly developed as at present. 



The Silurian limestones of America, Asia, and 

 Europe differ very little in their general fades of 

 organic remains. You have no difficulty in recog- 

 nizing the old features which struck you when ex- 

 amining the Dudley strata ; but when more 

 minutely studied, the naturalist makes out certain 

 " colonies," caused doubtless by difference of geo- 

 graphical circumstances. As the time passed away 

 during which the great sequence of beds belonging 

 to the Silurian formation were being elaborated, 

 other changes took place in organic life. The most 

 marked feature was that of a progression from 

 lower to higher types. Species multiplied, and the 

 general facies of life became more varied and less 

 cosmopolitan. 



The lowest beds of my parent formation go by 

 the name of Llandeilo Elags, so named from the 

 locality in North Wales where the typical section 

 may be studied. They are, as their name implies, 

 strata of flaggy sandstone, much worked for com- 

 mercial purposes. There is a considerable quantity 

 of limy matter in their composition, and this gives 

 them a peculiar endurability. Interstratified with 

 the beds of this deposit are immense layers of 

 ancient volcanic matter, — basalts or tuffs: these 

 flowed over the old sea-bottoms, when ejected 

 from submarine volcanoes, or volcanoes situated 

 near to the coast, as we find they usually are now- 

 adays. The ashes or tuffs were carried by the 

 winds, and the ancient seas had their surfaces 

 thickly strewn with cinders for hundreds of miles ; 

 these sank to the bottom, and alternated with the 

 regular shore deposits. Succeeding these Llandeilo 

 Elags, we have another division, known as the Bala 

 Limestone, also named from a locality : it has in- 

 terstratified with it beds of sandstone, slates, and 

 volcanic rocks again, which were doubtless strewn 

 over the old sea-bottoms just like those already 

 mentioned. The Caradoc Sandstones, named from 

 their locality in Shropshire, containing also shelly 

 sandstones, with soft shales and conglomerates, lie 

 above the Bala Limestone, and complete what 

 geologists have termed the "Lower Silurian Rocks." 

 They differ, as a whole, in Great Britaiu, from their 

 comprehending such a huge bulk of strata of igneous 



or volcanic origin. In some places these are 

 actually thicker than the rocks of sedimentary 

 origin. What a stormy, restless epoch was that ! 

 The old sea-bottom was subjected to shocks and 

 volcanic overflow more intense than those in the 

 neighbourhood of Iceland, where the Skaptar-jokul 

 is quiverins: with suppressed rage and superfluous 

 power ! Then, again, these Lower Silurian rocks 

 have neither so abundant, nor so highly organized a 

 fauna as the rocks of later date. 



Let me mention the next in order, before I give 

 you my personal recollections of the extinct 

 creatures you find imbedded in these rocks as 

 fossils. The "Middle Silurian" strata commence 

 with the Llandovery slates (another localism) ; 

 after which you have the May Hill sandstones 

 (about which not a few geologists quarrelled some 

 years ago) and the Tarannon shales ; altogether, 

 this series is about two thousand feet in thickness, 

 the Lower Silurian beds I have described being 

 upwards of nineteen thousand feet thick. Next 

 come the uppermost beds (to which I personally 

 belong), known as the " Upper Silurians," and 

 which attain a total vertical thickness of nearly 

 five thousand feet. They include several deposits 

 of minor importance ; such as the Woolhope beds, 

 and the Wenlook limestones and shales, completing 

 what is known as the " Wenlock Group." Then 

 succeed the Ludlow beds, of Aymestry limestones, 

 and the Downton sandstones, in the latter of 

 which is found a bed composed of scarcely any- 

 thing else but the bones, teeth, and scales of small 

 fishes, belonging to the placoid and ganoid orders. 

 It is in these soft shales you find the fossils so well 

 preserved. The shells, although they have been 

 extinct for unknown millions of years, still retain 

 their beautiful iridescent nacre," which, however, 

 soon decomposes by atmospherical influence. 



So much for the "stratigraphy" of this most 

 interesting geological formation ! At the forms of 

 life which swarmed the seas of this distant epoch 

 I cannot do more than merely glance. I have 

 mentioned that, generally speaking, there was a 

 progression. This is true only of the advance in 

 the main, for, during the earlier portions of the 

 Silurian period, huge Orthoceratites abounded, and 

 these are among the highest classes of the mollusca. 

 The muddy sea-bottoms swarmed with " sea-pens," 

 now known as Graptolites. But, of all forms 

 of life, those of the Trilobite family were most 

 abundant. Several hundred species are known to 

 belong to the Silurian formation alone. They 

 were crustaceans of that family to which the 

 lobsters and crabs of the present day belong, and 

 which has always been noted for its aberrant types. 

 Like other Crustacea, they underwent meta- 

 morphoses and changes. So well do the old rocks 

 tell their story of ancient life, that the geologist 

 has traced the metamorphoses of Trilobites through 



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