C. LAPWORTH ON THE MOFFAT SERIES. 333 



fauna, again, is even more strikingly contrasted. Although, composed 

 throughout of the same zoological groups, it shows a continued and 

 most important change in its aspect as we follow it from the base to 

 the summit of the formation. Not a single species is known to range 

 through more than half its total thickness. Nowhere is there any 

 adequate aj>parent cause for this extraordinary effect. Nowhere is 

 there any reason for doubting that it is due to those gradual and 

 cumulative changes brought about in every extended period of time. 



In their general mineralogical features, and in their rapid varia- 

 tion in palseontological characteristics, the rocks of the Moffat Series 

 are very different from any of the typical Silurians of the Princi- 

 pality. Among Palaeozoic formations the only strata with which in 

 these respects they admit of satisfactory comparison are the dark 

 shales imbedding the calcareous rocks of Scandinavia, where the 

 enormously thick Silurians of Wales are represented by less than 

 2000 feet of fossiliferous rock, and where each fossil has but a few 

 feet of vertical range. Among Secondary deposits they may be 

 paralleled with such formations as the Speeton Clay of the coast of 

 Yorkshire, where the massive Upper Jurassic and Neocomian rocks 

 of the Continent and the south of England are represented by less 

 than 500 feet of dark shales and clays, in which each zone is 

 marked by its peculiar Ammonite, and each fossil ranges through 

 but a small fraction of the entire succession. 



If these analogies are allowed their due weight (and every new 

 fact gathered in the Moffat district adds something to their force), 

 they inevitably lead to the deduction that the Moffat Rocks are the 

 greatly attenuated representatives of enormous thicknesses of the 

 Welsh Silurians, in which their fossils will be found to have a vastly 

 extended range. In other words, there is a high probability that 

 the Divisions of the Moffat Series have, in truth, a systematic im- 

 portance equal, or at least approximating to that of the so-called 

 formations of Siluria. 



§ II. Comparison of the Faunas of the three Divisions of the Moffat 

 Series with those of their Foreign Equivalents. 



Owing to the great rarity in the Moffat Series of fossils belonging 

 to the well-understood families of the Brachiopoda and Crustacea, 

 which are universally regarded as the most trustworthy exponents 

 of the geological age of their containing beds, we are forced to rely 

 almost exclusively upon such evidence as may be afforded by their 

 Graptolithina. In our comparative ignorance of the relationships 

 and vertical distribution of the forms composing this peculiar group, 

 all testimony derived from this source would, formerly, have been 

 either wholly ignored by the geologist, or received with grave sus- 

 picion. Within the last few years, however, a sufficiency of evidence 

 has gradually been accumulated, which places it beyond a doubt that 

 these fossils are quite as restricted in their geological distribution 

 as those of the better understood zoological groups. In the face of 

 the complete, and indeed overwhelming, proofs of the truth of this 

 fact, which follow from their vertical arrangement in the different 



