as Hems of Geological Time. 337 



from the Caradoc Sandstone only about 11 per cent, pass onward into 

 the Upper Silurian strata. These phenomena indicate gaps in geolo- 

 gical time unrepresented in the Silurian series of Britain by stratified 

 deposits, and, therefore, also unrepresented by genera and species, that, 

 did we know them, might serve to link together the life of the uncon- 

 formable formations in a more graduated succession of forms. I recapi- 

 tulate these opinions, -which were in part originally given in my first 

 Presidential Address to the Geological Society (1863), because they bear 

 on the arguments that follow. 



Like the Cambrian and Silurian rocks, the Devonian strata have also 

 been classified in three divisions by palaeontologists Lower, Middle, and 

 Upper. In Britain the Lower Devonian fauna is poor in numbers, 

 while it is rich both on the continent of Europe and in North 

 America. In. England both the Middle and Upper Devonian fossils are 

 plentiful enough. According to Mr. Etheridge, out of 74 English forms 

 25 per cent, pass from the Lower into the Middle division ; while, out of 

 268 forms, 25 per cent, pass from the Middle into the Upper Devonian 

 strata. No one has yet proved that these breaks in palaeontological 

 succession in the Devonian strata are accompanied by unconformable 

 stratification ; but the entire region has never been accurately mapped 

 according to the detailed methods of modern work. However this may 

 turn out, the vast thicknesses of these strata, characterized, like the great 

 Silurian divisions, by three marine faunas, of which the species are 

 mostly distinct, would seem to indicate that the time occupied in their 

 deposition may be fairly compared with that occupied in the accumula- 

 tion of the Silurian series. 



I accept the view that the Old Red Sandstone, as a whole, is the 

 general equivalent in time of the Devonian formations, and probably of 

 a good deal more ; for our Lower Devonian beds have no defined base, 

 and, therefore, their precise relation to the British Upper Silurian rocks 

 is unknown, whereas the Upper Ludlow rocks of Wales and its borders 

 pass conformably, and somewhat gradually, into the Old Eed Sandstone. 

 If the Devonian rocks be the equivalent of the Old Eed Sandstone, it 

 follows that the time occupied in the deposition of the latter may have been 

 as long as that taken in the deposition of the Cambrian and Silurian series. 

 This position is greatly strengthened by the thorough specific, and in 

 great part generic, differences in the fossils of the Upper Ludlow and 

 those found in the marine Carboniferous series differences that, to my 

 mind, indicate a long lapse of time, represented by the deposition of the 

 marine Devonian strata, during which time the Old Eed Sandstone was 

 being elsewhere deposited in the large lakes of an ancient continent. 

 These palseontological comparisons seem to me to indicate the vast 

 length of time necessary for the accumulation of these old lacustrine strata. 



The next question to be considered is, what time the deposition of 

 the Old Eed Sandstone may have taken, when compared with the time 



