242 TKANSACTIONS AND PKOCEEUINGS OF 'i II K [Sess. Lxvi. 



which have been found under marine conditions, may be 

 judged of by the percentage of extinct to living species of 

 mollusca occurring in each — the comparison Ijeing made 

 with reference, of course, to the species living in the 

 respective adjoining seas. In this way a geologist would 

 say that such or such a deposit in the ^Mediterranean 

 area is contemporaneous with another such in Britain, 

 because the percentage of extinct species of mollusca, as 

 compared with those now living in the two respective 

 areas, is identical. Measured by this standard, the oldest 

 part of Etna is of about the same age as the Forest Bed. 

 Now, after these strata beneath Etna were laid down, the 

 whole of that vast pile, 10,000 feet in height and 90 

 miles round, has been built up. Carefully observed data, 

 obtained on Etna, point to that volcano having grown at a 

 rate of about one foot in three hundred years. I should 

 be inclined to set the rate at a much slower one even than 

 that. Taking that, however, as a measure, the date when 

 the Newer Pleiocene rocks there (and here) began to be 

 formed would appear to be not less than three millions of 

 years back in the past. Independent evidence of other 

 kinds, which there is not room to give here, confirms this 

 estimate. Assuming, for the sake of argument, that these 

 figures are approximately correct, we see that the changes 

 which have affected western Europe during the last three 

 million years have sufficed for the extermination of a large 

 number of mammalia, and of some invertebrata also ; but 

 these changes, great as they have been in this time, have 

 not sufficed for the extermination of a single plant. This 

 fact is worthy of very special consideration, not only from 

 the point of view of the biologist, but for that of the 

 geologist as well, on account of its bearings upon the 

 question of the Age of the Earth. 



Overlying this ancient Forest Bed, and therefore of 

 later date than that deposit, occurs a marine deposit con- 

 taining Ostrea, and an arctic mollusc, Leda myalis. This 

 stratum in its turn, is in places succeeded by wliat is called 

 the Arctic Fresh-water Bed, which contains Salix polaris, 

 Betula nana, and the remains of the Pouched Marmot — 

 Hpe,rm.oi~)hilvs. 



Then follows a grand display of rocks formed under 



