as items of Geological Time. 339 



The change of life in the sea-bottoins was, so to say, partly local, and 

 due more to minor accidental physical causes than to that larger kind of 

 change that is marked by great disturbance of a lower set of strata, 

 long-continued denudation, and the subsequent unconforrnable deposi- 

 tion of a newer set of beds upon them, thus clearly indicating a long 

 lapse of time unrepresented by stratified deposits over a given area. I 

 therefore infer that the whole of the Liassic and Oolitic series must be 

 looked upon as presenting the various phases of one fades of marine life, 

 belonging to one geological epoch, marked by boundaries below and above 

 which depended on definite physical conditions over a large area. Such 

 a state of things in this Mesozoic epoch is comparable to the changes in 

 the fossil contents of the various subformations of the Cambrian and 

 Lingula-flag series, of which the Tremadoc slates form, an upper member ; 

 and, in my opinion, the comparison holds good even partly in the 

 manner of their deposition, parts of both series having been locally 

 deposited in waters not marine. On these grounds, therefore, the 

 Jurassic formations, as a whole, may be compared with these early Palaeo- 

 zoic formations in the length of time occupied for the deposition of each. 



If this inference be just, then, in like manner, they may be compared 

 with the Lower Devonian strata in England poor in fossils as far as is 

 yet known, but rich on the continent of Europe and in North America ; 

 and this (assuming that the Devonian and Old Eed Sandstone strata are 

 equivalents) implies that a lower portion of the Old Red Sandstone may 

 have talc en as long for its deposition as the wliole of the time occupied in the 

 deposition of the Liassic and Oolitic series. 



It is now generally allowed that the "Wealden beds of England are the 

 freshwater and estuarine equivalents of the Lower and Middle ]STeo- 

 comian strata of the Continent, which, in a palaaontological sense, may 

 be said, in some degree, to be related to the uppermost Jurassic strata, in 

 so far that a certain proportion of the species of Mollusca are common 

 to both, as shown by Forbes and Godwin- Austen ; while, in our own 

 country, from the Lower Greenland (Upper Neocomian) about 14 or 

 15 per cent, of the fossils pass on into the Upper Cretaceous strata. 

 The same kind of proportion, but in less degree, is found in the relations 

 of the Tremadoc to the Llandeilo and Bala series, and of the latter to 

 the Upper Silurian formations, and also of the Lower to the Middle, and 

 of the Middle to the Upper, Devonian strata. Those last named being 

 representatives in time of parts of the Old Eed Sandstone, it follows 

 that the whole of the time occupied in the deposition of the Old Red Sand- 

 stone may have been equal to the wliole of the time occupied in the deposition 

 of all the Jurassic, Purbeck, Wealden, and Cretaceous strata collectively. 



The next term of the continental era under review is the Carbonifer- 

 ous epoch, which, in its various conditions and numerous local subdivi- 

 sions, may with considerable propriety be compared to the Eocene period. 

 The deposits of both are locally of marine, estuarine, freshwater, and 



