XX INTRODUCTION. 



if it were otherwise, since Nature knows no units, and boundary 

 lines in conformable deposits are purely utilitarian or academic/' 

 Nevertheless, the academic boundary is well indicated in the 

 contrast between the Lower Greensand and the Wealden floras 

 of this country. In 1913 Halle wrote, in relation to the 

 Mesozoic flora of Patagonia: "It must be stated at once that 

 it is not possible, with the present state of the knowledge of 

 fossil floras, to establish any accurate subdivision, on a pala3O- 

 bofcanical basis, of the time from the close of the Jurassic to the 

 Lower Albian. The Wealden flora, as understood by palseo- 

 botanists, embraces more or less the whole of this time, several 

 of the characteristic Wealden species being found as high as in 

 the Albian of Portugal (Saporta, 1894:)." The present work on 

 the Aptian affords great hope that, as the structures of plants 

 of this age are discovered in various localities, the pala3obotanieal 

 divisions between the Jurassic and the Lower Albian will be 

 established. 



To the Jurassic, our Aptian flora, with its preponderance of 

 Conifers and its Dicotyledons, offers a contrast indicative of the 

 passage from one major life-sequence to another. Xcvertholess, 

 some of the differences in composition between the Wealden 

 and the Lower Greensand floras are due to the different physical 

 circumstances of their deposition. For instance, the absence of 

 fern-foliage in the Lower Greensand must be due simply to the 

 destruction of the leaves before they reached a position in which 

 they could be entombed. 



Leaving the Coal-Measure flora out of consideration, as it is 

 unique in many ways, it may generally be stated that the floras 

 of which any considerable numbers of forms are known are 

 preponderatingly composed of species based on fragments of 

 ladies. The Lower Greensand flora, however, is preponder- 

 atingly composed of woody stems, with a fair sprinkling of 

 gymnospermic cones. The reason for this is doubtless corre- 

 lated with the palaeogeography of the deposit. Everything 

 points to the Aptian deposits of this country as representing a 

 narrow arm of the sea, in which a coarse marine detritus was 

 being laid down at no great distance from land. The plant- 

 remains which were mingled with the coarse sandy matrix 

 must have drifted for some time before they were entombed. 

 While sea-water has proved to be an exceedingly good preser- 



