44 INTRODUCTION. 



with the rocks in which occur any fossils that they may have to 

 examine and describe ; and many errors have arisen from the 

 neglect of this sound rule. 



Again, it is now clearly recognised that in any comparison between 

 two sets of beds by means of their fossils, with a view of ascertain- 

 ing their relative age, it is essential to take into account the condi- 

 tions under which the deposits in questio?i were formed. Thus, two 

 marine deposits, both accumulated in shallow water, can be fairly 

 and fully compared with one another, but they can only be imperfectly 

 compared with a deposit formed in a deep sea, and they cannot 

 be compared at all with deposits which have been formed in inland 

 fresh waters or on land. Hence we find that palaeontologists have 

 differed, and still differ, widely with regard to the relative value as 

 tests of the age of strata, to be assigned to different classes of fossils. 

 In certain cases — some of which will be more fully referred to sub- 

 sequently — a different age would be assigned to a group of beds on 

 the strength of its vegetable remains to that which would be deduced 

 from a study of the animal fossils of the same or of associated beds. 

 Or, the marine Invertebrate fossils in some cases may point to one 

 age for the beds, while the remains of Vertebrates indicate another. 

 Such cases must be dealt with on the following general principle : — 



Deposits containing numerous land-plants or the bones of terres- 

 trial Vertebrates are mostly estuarine, lacustrine, or fluviatile in 

 origin, though in some cases they have been formed on land. On 

 the other hand, deposits containing marine Invertebrates have been 

 laid down in the sea itself, for the most part at moderate depths. 

 There is, however, absolutely no reason for thinking that the succes- 

 sion of life as regards the land-plants and land-animals of any given 

 region has been in any way parallel with the evolution of the marine 

 animals of the seas of the same region. The evolution of the ter- 

 restrial organisms may have been much more rapid or much slower 

 than that of the marine forms. Hence, it is quite possible that the 

 land-plants and land-animals which are regarded as characteristic of 

 a particular geological period, may be found coexisting with marine 

 animals which are considered to characterise a different geological 

 period. If, however, we take the stratified formations as a whole, 

 we find that it is in the main by means of their marine faunae that 

 their relative age has been determined, the reason of this being the 

 obvious one that the great majority of stratified deposits have been 

 laid down in the sea, and the record of the succession of terrestrial 

 organisms in time is a very incomplete one. The general chrono- 

 logical standard, as based upon palseontological evidence, is, there- 

 fore, founded mainly on the results afforded by the marine Inverte- 

 brates. In those cases, therefore, in which the marine deposits of a 

 given series of beds would indicate through their Invertebrate fossils 



