194 Proceedings of the Royal Physical Society. 



quires careful and cautious consideration is the systematic 

 disposal of what have been termed " passage-beds," or strata 

 that lie between and partake of the characters of two con- 

 tiguous systems. There are, for instance, passage-beds be- 

 tween the Silurian and the Old Bed, between the Old Eed and 

 the Carboniferous, between the Trias and Lias, and in all 

 likelihood between most of the other great stratified sys- 

 tems. The final disposal of such strata involves the ques- 

 tion, What constitutes a system ? -Shall we regard a system 

 as a great series of strata characterised by the same facies of 

 organic remains ? or shall we consider a system as bounded 

 on both sides by great physical breaks or unconformities in 

 stratification? If we adopt the former . view, the line be- 

 tween systems must often be vague and uncertain ; and if 

 we adhere to the latter, we find that physical breaks are not 

 always followed by an immediate and total change, either 

 in the flora or fauna. The truth is, that in this, as in other 

 geological questions, we must adopt a somewhat provisional 

 course, avoiding sharp lines of demarcation, and using the 

 term " passage-beds" where neither mineral nor fossil cha- 

 racters are decided enough to lead us to a conclusion. Were 

 we to adopt the views of some, the lowermost Old Eed flag- 

 stones of Scotland would rank as uppermost Silurian ; while 

 the uppermost yellow beds would go to form the base of the 

 Carboniferous formation. In this way the Old Eed Sandstone 

 of Scotland would be reduced to mere subordinate forma- 

 tion, and this without rendering more intelligible the boun- 

 dary between Murchison's Siluria and Devonia on the one 

 hand, or giving to the officers of the Irish Geological Survey 

 a surer basis for their Carboniferous system on the other. In 

 the same way with the Ehaetic and Penarth beds that lie 

 between the Trias and Lias. In some districts the fossil 

 assemblage seems to point to the Lias, and in others to the 

 Trias, and where there is no physical break it matters little 

 to which system we assign them. But where a physical 

 break or unconformability occurs, that break should be held 

 as the boundary between the Triassic and Jurassic systems. 

 And, more than this, where such questions cannot be solved 

 by the examination of British strata, we should appeal to the 



