Geology of the Neighbourhood of Weymouth, Sgc. 17 



Portland Stone. 



As the details of the component beds and fossils of the Portland series 

 have been sufficiently described by Mr. Webster and Mr. Conybeare, we shall 



of an erect stump protruding through the burr stone into the superior stratum called Aish, 

 analogous to that represented by Mr. Webster in his section of the Portland beds (Geol. Trans, 

 Second Series, vol. ii. p. 1. Plate VI. fig. 3 and 4.), and which the workmen stated to be of rare 

 occurrence ; this superior stratum exhibited to Prof. Henslow two or three of those conical and 

 dome-shaped protuberances which so generally indicate the presence of stumps of trees imme- 

 diately beneath them. In the case represented, the protruding stump being longer than the rest, 

 the burr stone was not deposited in sufficient thickness to cover it, but was heaped around it in 

 two circular ridges, with intermediate circular depressions. From these appearances we may infer 

 that a state of great tranquillity attended the deposition of the calcareous sediment of which the 

 dome-shaped caps that cover the shorter stumps were formed (See Section of the Dirt-bed, 

 page 13.) The circular ridges and depressions which surround the taller stumps (see page 16,) 

 seem to have been produced by an interruption of the undulations on the surface of water so 

 shallow that the waves were caught and broken by a stump but four feet high, whilst they passed 

 over the shorter stools immediately adjacent to them ; these interrupted undulations being propa- 

 gated first downwards and then outwards along and around the stump whose top was high enough 

 to obstruct the ripple on the surface of the shallow lake, by which the dirt-bed was gradually 

 inundated. 



Here, then, we have proofs of a tranquil state and gentle action of water in the period imme- 

 diately succeeding, as well as in that which preceded, the formation of the dirt-bed, upon a sur- 

 face that became dry land during a short time intermediate between the transition of the district 

 which it covers from a submarine state to that of a freshwater lake. The rapid possession wliich 

 in our modern tropics is taken by the Pandanus or Screw Pine and Cocoa-nut Palm of the first 

 banks and reefs of coral islands that emerge above the level of the sea, affords an example of the 

 luxuriant growth of vegetables on the margin of land just rising above the water, analogous to 

 the ancient juxtamarine forest the remains of which contributed to the formation of the dirt-bed, 

 in the region which has now become the southern coast of Dorsetshire. 



With respect to the silicified trees Prof. Henslow makes the following observations : — 



" From what I saw I should think that all the erect stumps must have suffered considerable 

 decay before they had become imbedded, or at least fossilized, in the burr. They consisted of no 

 more than the central portion of the wood just above and below the neck of the trees, which had 

 every appearance of having grown in the places which they still occupy, 



" In a quarry of very white and chalk-like Portland stone, at the base of Chalbury Hill, near 

 Preston, I found a cylindrical mass of flint ten inches in diameter, reposing upon a soot-like mass 

 of carbonaceous matter, probably resulting from the decomposition of leaves and bark, and 

 forming an envelope to the lower part of the cylinder, but not extending beyond it. Upon crack- 

 ing off as much of this cylinder as protruded from the side of the quarry, I perceived the central 

 portions, of three inches in diameter, to consist of fossilized wood. It appears to me most pro- 

 bable that the whole cylinder had occupied the space originally filled by the trunk or branch of 

 a tree ; but that during the process of its becoming silicified, the organic structure of the outer 

 rN portions had not been impressed upon the flint. In the Isle of Portland the quarry-men collect a 

 ■^ similar black vegetable substance, which they use for blacking." 

 rJ VOL. IV. — SECOND SERIES. D 



