220 Dr. FiTTON on the Strata below the Chalk. 



be more remarkable than the immediate contact of these two groups ; the upper consisting of 

 freshwater limestone with few fossils; the lower distinctly oolitic, with fossils in great number 

 and variety, and all of them marine*. 



3. Besides the " Black Dirt" (No. 8.), or, as it is usually called by the quarry-men, " the Dirt ", 

 two or more thin courses of clay occur above the Cap: one (No. 3.), about two inches thick, 

 between the Bacon-ledge and the Slate; another (No. 5.), about an inch in thickness, dividing 

 the former from the Ash. The number, however, of these thin beds of clay varies in the different 

 quarries ; and, in general, it may be said that such alternations with the limestone are not unfre- 

 quent. They all bear the name of " Dirt", and are more or less mixed with fragments of stone 

 and with carbonaceous m.atter; but contain commonly much more clay than either of the two 

 beds hereafter mentioned, which include the remains of plants. 



4. The Bacon-tier consists of calcareous slate alternating in some places with thin beds of sand. 

 It is from 2 feet to 2^ feet in thickness. 



6. The Ash is a soft fissile limestone, like the last-mentioned bed, from which it is easily sepa- 

 rated in working. In general it is from eighteen inches to two feet thick, and is closely connected 

 with the Soft Burr, 7. When the incumbent beds are removed, it generally exhibits an uneven 

 surface, with numerous bosses or prominences, enveloping the broken tops of the trunks, which 

 stand upright in the dirt-bed below, as described by Dr. Buckland and Mr. De la Beche. 

 Oblong depressions also are frequently observed on the surface of the Ash, called by the quarry- 

 men " graves ", the origin of which it is not easy to explain. 



7. The Burr of the Ash, or Soft Burr, is between two and three feet thick. It has obviously 

 been deposited around the lower part of the petrified trunks, and is always separated from them 

 by a small space, at present occupied by the carbonaceous matter produced by the decomposition 

 of part of the tree : and this is the case also where the Ash envelopes the top of the broken 

 stumps. The junction of these two beds, around the trunks, is often attended with some very 

 interesting appearances, for an account of which I refer to a note by Professor Henslow in 

 the preceding part of this volume f. 



8. The "Dirt", or "Black Dirt", in Portland is separated from the stone, both above and 

 below, by well-defined surfaces ; while some of the other beds of clay, or dirt, pass by gradation 

 into, or adhere closely to, the stone which adjoins them. It is from twelve to eighteen inches 

 thick, and differs from all the other beds alternating with the slaty limestone, in containing 

 large worn fragments of stone j, from three to nine inches in diameter, in such numbers that 

 the whole deserves the name of coarse gravel. With these are mixed coarse carbonaceous 

 matter and minuter fragments of stone ; but seldom, in the Isle of Portland, any continuous or 

 cohesive clay : — the carbonaceous matter arising, no doubt, from the remains of vegetables, and 

 being most abundant immediately around the trunks and Cycadeas, which this bed includes, or 

 supports. 



At Upway, on the north of Weymouth, the representative of the " Black Dirt" likewise con- 

 tains petrified trees : but the pieces of stone which it includes are few and much smaller, and 

 it approaches more nearly to the usual form of vegetable mould, or of clay. On the north-east 

 of Weymouth its place can be distinctly recognised in the cliffs between Lulworth Cove and Wor- 



* Compare what is here stated, with the account of the junction of the marine beds with the 

 top of the Wealden, at Atherfield in the Isle of Wight {supra, p. 196, last lines). 



t Pages 16. and 17. 



X In these fragments I could find no fossils ; Mr. Webster considers them as belonging to the 

 lower beds of the Portland stone. 



