THE OOLITIC SYSTEM. 727 



metropolis. The isle of Portland, where this stone is obtained chiefly, is, properly 

 speaking, a bold promontory off Weymouth, about four miles and a half in length, and 

 two in breadth. Its base consists of Kimmeridge clay and the series of building-stone, 

 upon which the remarkable " dirt-bed " presents itself. This is a bed of black mould, 

 dark argillaceous mud, in which there exist large erect stumps of cycadese, or tropical 

 trees, which must have grown on the spot where the stumps now stand, broken as if by a 

 hurricane. The following conclusions respecting alternate elevations and subsidences of 

 the strata in this district seem to be fairly inferrible from the position of these vegetable 

 remains: 1. The oolitic limestone, or Portland stone, a marine formation beneath the 

 dirt-bed, was deposited at the bottom of the ocean. 2. This formation rose, either 

 gradually or violently, till it reached the surface of the sea, and became dry land for a 

 time long enough to allow of the accumulation of the black mould, and the growth of the 

 trees. 3. The surface was afterwards submerged beneath the waters of a river or a 

 fresh-water lake, as the superjacent strata are of fresh-water formation. 4. The whole 

 has subsequently been elevated into the position now occupied by the strata in the hills 

 of Dorset. However at variance with the present comparatively quiescent state of the 

 earth these great geological changes may seem, we are compelled to have recourse to 

 them to explain the phenomena ; but supposing them to have gradually transpired the 

 discordance vanishes, for precisely parallel examples of elevation and subsidence have 

 marked the operations of nature during the brief history of mankind. 



In stating the range of the oolitic system in England, including the lias, it has been 

 described as extending in an almost uninterrupted band from the coast of Dorsetshire, 

 through the midland and eastern districts of the kingdom, to the sea-coast of Yorkshire 

 above Whitby ; and though the subordinate divisions of the oolite are identified by local 

 names chiefly with southern districts, they must not be supposed to be confined to them. 

 Thus the great oolite, or Bath stone, is a formation prominent about Northampton and 

 Stamford, where it is quarried, and extends continuously into Yorkshire, though there it 

 is covered by the chalk. The coralline oolite is exposed between Scarborough and 

 Malton, and at the latter place it becomes likewise concealed by the chalk. The Oxford 

 clay also is met with to the north of Lincoln ; the Portland stone appears in the Vale of 

 Aylesbury, and the Kimmeridge clay fills the Vale of Pickering, watered by the York- 

 shire Derwent. An intelligent observer of the oolitic formations of the north, Dr. Young 

 of Whitby, remarks upon the numerous vertical fissures which occur in the strata 

 serving as conduits for the waters which descend upon the superficies. Most of the rain 

 that falls on the hills around the Vale of Pickering is immediately absorbed by the light 

 soil that covers them, and sinking into the fissures, it runs through the hills in subter- 

 ranean streams, which at length burst out at the foot of them in copious springs, some- 

 times in actual torrents. Hence these hills are remarkably dry. A spring is seldom 

 found in any elevated situation, and wells there require to be dug to an immense depth to 

 find water. Hence, also, as all the springs break out at the base of the hills, where they 

 descend into the Vale of Pickering, a chain of towns and villages skirts their foot, built 

 at these springs for the sake of their copious waters. Another common phenomenon of 

 the district is also explained by these clefts in the rocks. The fissured hills not only 

 absorb their own waters, but swallow up the rivers and streams which pass through their 

 intervals from the country beyond them ; for these currents sink into the fissures, and 

 flow under ground, till they disembogue in the low Pickering Valley, where they burst up 

 again as new springs. Yet still a channel remains above ground also, to convey the sur- 

 plus waters, which the subterranean course, in some parts of the year, is incapable of 

 admitting. Dr. Young notices several examples of this subsidence and reappearance of 

 streams in the neighbourhood. 



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