226 Dr. FiTTON on the Strata below the Chalk. 



perhaps so gradually as to have been near the surface for some time before 

 its emersion. The next step, in the series of revolutions which produced 

 these strata, must have been the diffusion of fresh water over the new land, 

 from which the " Skull-cap " was deposited ; and this was followed by a 

 drying up or retirement of the water, so as to disclose and convert into dry 

 land the lower of the two beds of clay, or dirt, which now contain the Cy- 

 cadeae. I am not sufficiently acquainted with the habits of the recent plants 

 of this family, to form any opinion as to the length of time necessary for 

 their acquiring the size of the specimens now found petrified, nor to judge 

 how far the rapidity of their growth would be modified by climate : but 

 all the known species are said to be of slow growth; and being found at 

 Japan, as far north of the equator as 40°, and at the Cape of Good Hope 

 and Port Jackson, in the southern hemisphere, about latitude 34°, they cannot 

 be considered as strictly tropical productions. 



A second submersion of the surface in fresh water must have followed the 

 production of the Cycadeae in this lower bed, from which the "Cap" was accu- 

 mulated, to a thickness of between seven and ten feet, and the " Black Dirt" 

 deposited above it; and a third, and apparently more durable submersion, still 

 in fresh water, must afterwards have taken place, by which the " slate " and 

 a great part of the Purbeck strata were produced. In this last case the water 

 seems to have been at first unmixed, but to have become after a time ac- 

 cessible to the sea; since not only does the Purbeck formation contain a 

 thick bed of oyster-shells, but thence upwards, throughout the Wealden, 

 oysters are found, in strata which in several instances alternate with others 

 abounding in freshwater shells. 



(114.) Mr. Ly ell has pointed out the resemblance between the geological 

 events which produced the strata of Portland Island, and those which are 

 known to have occurred at the mouth of the Indus, from the effect of suc- 

 cessive earthquakes connected with volcanic eruption*; and it is clear that 

 all the changes above supposed to have taken place, — depression and elevation 

 of the land, through comparatively small depths and at different periods, witn 

 alternate though irregular submersions both in salt and fresh water, — have 

 been produced in that region, not only within the period of tradition, but 

 many of them so recently as in 1819. Among other facts, it is stated, that 

 for some years after the earthquake of Cutch, which happened in that year, 

 " the withered Tamarisks and other shrubs protruded their tops above the 

 " waves, in parts of the lagoons formed by subsidence, on the site of the 



* Principles of Geology, 4th edition, vol. ii. p. 237-242. 



