Dr. FiTTON on the Strata below the Chalk. 225 



At Upway, this bed, or one of those immediately below it, is beautifully white, like chalk, and 

 contains nodules of dark grey flint, frequently hollow and lined with quartz crystals, which it 

 would be impossible, without attending to their fossils, and their geological position, to distin- 

 guish from those of the chalk. One of the characteristics of the best Portland stone, both of this 

 and other beds, is that the blocks ring very distinctly under the hammer, giving out a clear and 

 agreeable note. 



(112.) It appears, therefore, that on the Dorsetshire coast the Portland 

 strata are everywhere succeeded by a series of beds of freshwater limestone, 

 alternating with beds of clay, or of a compound called " dirt ", which con- 

 tains carbonaceous matter and fragments of stone ; and that two at least 

 of these "dirt" beds, the first about three feet, the second about twelve 

 feet above the top of the Portland series, include the remains of plants, which 

 grew in the places where they are found ; nor is it wholly improbable that 

 similar remains may hereafter be discovered in some of the other and thinner 

 beds of "dirt". 



A point which Mr. Brown considers as well deserving of remark is, that the 

 only remains of vegetables hitherto found in these strata, under the circum- 

 stances above described, belong to two nearly related families, Coniferae and 

 Cycadeae, which have lately been regarded as forming a distinct class, cha- 

 racterized not only by the greater simplicity of the parts of fructification, but 

 also by some peculiarities of internal structure, and thence have been con- 

 sidered as intermediate between Phanogamous, and Cryptogamous or Aco- 

 tyledonous plants. 



Another striking fact connected with the remains included in the "dirt 

 beds" is, that they are composed almost entirely of silex, though surrounded 

 either by limestone or by the mixed components of the dirt itself. The 

 cavities in the petrified trunks are lined with minute crystals of quartz ; and 

 Dr. Prout, who has been so kind as to examine specimens of the coniferous 

 wood taken from the confines of the Purbeck and Portland formations, in Port- 

 land, and other places to which I shall refer hereafter, — finds them all to consist 

 almost wholly of siliceous matter, with very slight traces only of carbonate of 

 lime and of iron ; some of the darker parts of a specimen, from the Vale of 

 Wardour containing also bituminous matter. The Cycadeae likewise are 

 described by Dr. Buckland as being almost entirely siliceous, "varying from 

 " coarse granular chert to imperfect chalcedony"*. 



(113.) The great abundance of small, worn, fragments of shells and stone 

 at the top of the Portland formation, is what might have been expected 

 on the surface of an island of small height, just protruded from' the sea, 



* Geol. Trans., 2nd Series, vol. ii. p. 398. 

 2g2 



