223 Dr. FiTTON on the Strata below the Chalk. 



At 2 feet from the extremity of the root, the greatest diameter is 18f inches, the transverse 

 diameter only 12|; at 4 feet, the diameters are 16^ and 11:|: inches; at 11 feet, 14f and 11^; 

 and at 18 feet, 15 and 9| inches. The right branch at its extremity, 11 and 4^ inches. 



The specimen here represented is much like that which is preserved in tlie Isle of Portland, 

 in general aspect and proportion ; but many of the stumps which I saw standing in the quarries 

 were much larger. One that I measured, — of which 5 feet stood upright from the lowest point 

 of the roots to the top of the broken trunk, — was 2 feet in one of its diameters, and 6 feet 9 inches 

 in circumference. Another, 5 feet 10 inches in total height, was about 2 feet 10 inches in diameter, 

 and 9 feet 4 inches round ; and I was assured that others had been found of much greater thick- 

 ness. All the larger trees that I saw were in the quarries on the north-west of the church; one 

 of which afforded also the specimen represented in the wood-cut, with several large Cycadese. In 

 many places I saw large branches lying prostrate, and partially immersed in the Dirt; — one of 

 them, above 3 feet long, within 2 feet of another portion, about 10 feet in length, and in the 

 same direction with it*. 



From the evidence afforded by thin transparent slices, both of the transverse and longitudinal 

 sections, which have been examined under the microscope f by Mr. Brown, the fossil trunks of 

 Portland are found to possess the characters uniformly belonging to coniferous wood: but it must 

 be observed that these characters are not absolutely confined to Coniferas. 



The Cycadece found in the " Black Dirt" are generally from 9 inches to a foot in diameter, 

 and about 9 or 10 inches high. I myself saw but one specimen, which had been recently taken 

 from its place, in this bed ; and it was stated by the quarry-men that on the west of the main 

 road from Chiselton to the church, they have been found, or noticed, only in the Dirt above 

 the Cap ; while on the east of the road, as I shall presently mention, they are certainly found 

 below that bed. In the eastern quarries, indeed, the workmen, who had not seen them elsewhere, 

 asserted that the Cycadeae were never found above the Cap ; but this I found afterwards to be 

 incorrect. In both situations, they are much less common than the coniferous trunks. 



9. The " Cap" is the thickest of all the strata above the Portland stone ; the average thickness 

 being about 8 feet, the extremes 6 and 9 feet; of which, in some cases, about 18 inches at the 

 top are easily separable from the rest. When recently exposed, the whole of the remaining mass 

 is continuous, and consists chiefly of uniform limestone of a light brownish or drab colour, with 

 a flat conchoidal and splintery fracture ; but it cannot be used for building, as, in the workman's 

 phrase, " it will not square." About the middle and towards the lower part it is spongy, or 

 cavernous, including tortuous cavities surrounded with botryoidal carbonate of lime ; and it has 



* It would be desirable to ascertain the direction in which the prostrate stems lie, with respect 

 to the meridian, and to the roots and portions of the trunk remaining upright, in order to deter- 

 mine whether they were overthrown by an uniform current of wind or water. 



t One of the longitudinal slices examined by Mr. Brown was broken from the extremity of the 

 left branch of the tree represented in the annexed wood-cut. 



The process by which these slices are prepared for the microscope has recently been carried 

 to great perfection by Mr. Nicol of Edinburgh. It consists in attaching a thin polished slice of 

 the fossil, separated by the ordinary method, to a piece of plate glass, by means of a uniform 

 cement ; and then grinding it down as far as possible on the lapidary's wheel. In this way tran- 

 sparent slices have been obtained, which exhibit the internal structure of fossil wood with beau- 

 tiful distinctness, and show the extreme delicacy with which the original vegetable structure 

 has been preserved during the petrifactive process. Two papers by Mr. Nicol on the structure 

 of the recent and fossil Coniferae are published in the Edinburgh Philosophical Journal, vol. xvi. 

 1834, pp. 137 and 310 ; and a third in the Report of the Fourth Meeting of the British Asso- 

 ciation for the Advancement of Science, 1834, p. 160. 



