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 Eed, 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 bo 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 
