xl PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



of Farley Castle and Bradford, with its Turbellaria, Apiocrinus, and 

 Terebratula coarctata, vanishes. So when the upper part of the Lias 

 dwindles in passing southward from Cleveland, we lose Leda ovum. 

 Amnion Lyiliensis, and Balanus tubidanis ; while, on the other hand, 

 as we proceed northward, and lose the Lower Lias shales, the bone- 

 bed vanishes. When the Lias Limestone grows obscure, Ammonites 

 BucTclandi and A. Conybearii cease; and this is the more striking, 

 because at intervals, some of the Middle and Upper Lias fossils, not 

 so limited in time, as A. bifrons and A. heterophyllus, reach the coast 

 of Dorsetshire, and reappear on the Continent. 



This unequal diffusion of definite forms of life may often be ascribed 

 to the progress of oceanic currents, which transported at once the 

 germs of life and the sediments in which they were buried. If we 

 trace by this means some of the ancient currents of the sea for any 

 particular epoch, we shall find, with surprise, some neighbouring tracts 

 to have been almost unconnected ; while very distant regions manifest 

 some effective communication. Thus, while the Trilobites of Bohemia 

 differ almost in every species from those of Scandinavia*, while only 

 a small proportion of the fossils of North Devon occur in South 

 Devon, the American genus Maclurea unexpectedly appears in the 

 oldest limestones of the extreme north-west of Scotland, and Ammo- 

 nites like those of Kelloway Bridge in Wiltshire are collected in the 

 centre of Bussia and at the mouth of the Indus. So in existing 

 nature, when we find Spirula in so many distant basins of the sea, 

 between which now are no connecting channels, we must appeal to 

 earlier distributions of land and sea for the means of intercourse 

 which no longer exist. Thus Nature in some material aspects retains, 

 in the arrangements of life as well as in the form of land and the 

 peculiarities of physical geography, traces of the history of an earlier 

 time. 



Another point which appears to be of great importance in tracing 

 the history of life, is the thorough examination of what are con- 

 ceived to be " passage beds" from one system to another. For ex- 

 ample, the Lingula-flags make a very remarkable zone in the series 

 of Lower Palaeozoic strata, separated by a considerable interval from 

 the more fossiliferous strata of Snowdonia. The dying-out of one 

 group of life, and the introduction of the other, in relation to the 

 mineral nature and structure of the masses, are worthy of special 

 attention in the vicinity of Tremadoc. In the group of the May Hill 

 Sandstones we trace very satisfactorily some of the circumstances 

 which characterize the introduction of the Upper Silurian fauna. 

 The Bev. W. Symonds has lately added some details of the right sort 

 in regard to the succession of beds, and their contents, at the junction 

 of the Old Bed Sandstone with the Siluriansf ; and Mr. Baily has 

 added to our knowledge of those " Upper Old Bed " laminations 

 which contain Cydopteris Hibernica and Anodonta Jukesii, and pre- 

 nunciate as it were the great Carboniferous system. To continue 

 this subject, I may recall to attention the very interesting junctions 



* Barrande and Murehison. 



f Read to the Geological Society, Nov. 2, 1859. 



