the Chalk, Greensands and Oolites. 191 



The dimensions and external form of the masses of chert in the greensand of 

 Lyme are widely different from those of the upper greensand, and would seem to 

 indicate a specific difference, but the resemblance of their internal structure evinces 

 a very close alliance in every other respect. Two specimens of chert from the 

 oolite, one being from the Tisbury limestone and the other from the Portland, 

 afforded similar strongly-marked evidences of organized structure ; but with this 

 difference, that in these there appeared to be a greater quantity of the cellular 

 structure of the sponge preserved than in any of the former instances ; and the 

 appearances afforded by the structure, in both cases, were nearer to those of the 

 freshwater sponge than any of the fossil sponges of the chalk or greensands, with 

 the exception that there were fewer spicula than in the recent freshwater species. 

 When small portions of dead and decomposing freshwater sponge were immersed 

 in Canada balsam, and examined microscopically in the same manner as the sili- 

 ceous slices, the appearance was so similar to that of the fossils, as to prove at once 

 that the oolitic cherts derived their origin from decomposed spongeous masses*. 



* With a view of confirming the above statement, I visited, during the summer of 1840, the Island of 

 Portland and the greensand formation at Blackdown. In the former district I found the principal source 

 of the numerous masses of chert abounding in the great talus beneath the cliffs of the western side of 

 the island to be an immense band of chert, which may be readily traced almost the entire length of 

 the western cliffs, and is situated about one-third from the upper surface, of the whole height of the 

 cliff that is apparent above the talus. This stratum varies from three to five feet in thickness, and 

 upon a close examination it is found to be composed, not of one uniform mass of chert, but of thick 

 tortuous branches, if I may be allowed the expression, which ramify and wind in all directions, leaving 

 intervals between them of nearly their own dimensions, and filled with the same material as that of which 

 the surrounding Portland stone is composed. This is decidedly not the form of a merely mechanical 

 deposit of siliceous matter, but it resembles closely, in every respect, the arrangement which large sponge- 

 ous masses assume when growing thickly strewed at the bottom of the ocean ; for although sponges, 

 under such circumstances, in their young state, are separate and distinct from each other, yet it is well 

 known, that it is their habit, when they grow to such a size as to approach and touch each other, to 

 unite and form vast masses, which spread over a great extent of surface. I examined the nume- 

 rous fresh vertical sections, frequently eight or ten feet in length, afforded by the masses which had 

 recently fallen from the face of the cliff, and in all these the oolitic material was observed permeating 

 the whole of the mass in an irregular and tortuous direction. In the sections of the chert thus examined 

 a white, clouded appearance was very generally observable, and upon examinination was found, in every 

 case, to be caused by the remains of a spongeous tissue, identical with that before described as occurring 

 in the specimens of chert formerly examined. This structure may be indistinctly seen with a good Cod- 

 dington lens, but to obtain an accurate idea of its nature, it should be viewed as an opaque object with a lei- 

 berkuhn, and an achromatic microscopic power of two hundred linear ; and it is only by such an examina- 

 tion that its true character can be observed with the greatest advantage. In the strata of Portland oolite, 

 both above and below this great bed of chert, large tuberous nodules of chert are frequently found. 

 These resemble very much in their external characters the irregular cherty nodules described as occurring 

 in the upper greensand of Fovant and other localities. The interior presents the same structure and ap- 



