Dr. FiTTON on the Strata below the Chalk. 211 



analogous to that of the sand and subcalcareous grit which occur between 

 the Oxford oolite and Oxford clay, — the inferior oolite and the lias clay: ?? 

 The whole mass of strata deposited in the first instance, having^ been in all 

 these cases, sand and mud; the upper part of which abounding more in calca- 

 reous matter, was subsequently converted into stone, and the lower more or- 

 less concreted into nodular masses of calciferous grit, in proportion to the 

 quantity of carbonate of lime diffused through it. 



In the Isle of Purbeck, the Portland sand first rises above the sea about midway between 

 Winspit and the point of St. Alban's Head, and is disclosed by denudation on the north of that 

 remarkable promontory, in a ravine called Pier Bottom, near which place the Kimmeridge clay 

 appears to rise on the shore. A good section of the sand is visible at Emmet's Hill, a ridwe 

 capped with Portland stone, between Pier Bottom and Chapman's Pool ; and the group may be 

 traced thence, beneath a capping of stone, all round the inflections of the high ground from 

 Bottom Farm, by the prominences called Encombe Point (or Hound's Tout), and Swyre Head, 

 through Kimmeridge, to Gad Cliff, where it declines along with the stone and sinks again under 

 the sea. Its relative place and its effect in modifying the surface, are well seen in the amphi- 

 theatre of high ground which nearly encircles Kimmeridge Bay ; of part of which PI. X. a. No, 9, 

 and X. b. fig. 6, are illustrations : the summits all around consisting of Portland stone, and 

 the rapid grass-covered slope beneath of this sand, resting upon a tract comparatively flat and 

 uniform, which is occupied by the clay of Kimmeridge. The sand again makes its appear- 

 ance on the coast west of Purbeck, in a small portion distinguishable by its bluish grey colour, 

 beneath the rocky cliffs called Horsewalls, which form the most prominent part of the shore 

 between Lulworth and Durdle Cove. In the complex and disturbed tract on the north and west 

 of Whitenore, the Portland sand is found wherever the superior strata rise high enough to dis- 

 close it ; and in the Isle of Portland it everywhere accompanies the stone, and has the same 

 characters as on the coast of Purbeck. 



The following is a List of the strata at EmmeVs Hill, immediately on the north of St. Alban's 

 Head :— PI. X. b. figs. 5 and 6. 



1. Portland stone, with layers of flint near the bottom, forming a shattered and inac- 1 

 cessible precipice at the top of the clift" 30 to 40 feet J 



2. Portland sand. Soft stone, or marly coherent sand, with a rugged external surface 

 of a yellowish grey or brownish colour ; but in the recent fracture, of a dark bluish 

 or greenish grey. It includes concretions of firmer consistence. In a bed of stone 

 of the same description at the cross-roads above Kimmeridge Farm, which seems to 

 belong to the same place in the series, Trigonia clavellata, Ammonites giganteus 

 and other fossils are found 



Feet. 



25 to 30 



3. Bluish grey, inclining to green, sandy clay and marl, including ranges of con- 

 cretions approaching to stone: the whole effervescing with acids. The more stony 

 portions acquire by exposure a yellowish brown tinge, and are in some places 

 coated with a crust of white pulverulent carbonate of lime. A bed of this group 

 dug for building near Bottom Farm, consists of bluish grey sandy stone, but [ about 60 

 slightly effervescent, and is streaked or mottled with portions of a darker hue. In 

 some other places the strata are divided by fissures into nearly vertical flakes, 

 two and three feet in thickness, and include occasionally nodules of much more 

 compact texture 



