266 Dr. PiTTON on the Strata below the Chalk. 



Purheck strata. — The only indication of this member of the Wealden which I found at 

 Swindon, were detached masses of botryoidal limestone, like the middle of the " Cap ", in Port- 

 land (p. 222.), and fragments of dark brownish silicified wood, — ^fallen, I have no doubt, from the 

 top of the quarries, thougii supposed by the workmen to occur amidst the stone itself. In the 

 looser matter also, near the top of the quarries and above beds of the Portland stone, are masses 

 including very numerous casts of marine shells (apparently Lucina and Cytherea), in which the 

 cementing substance has the aspect of freshwater limestone, and is scarcely to be distinguished 

 from a stone of the same yellowish hue, within the Purbeck series near Ridge in the Vale of 

 Wardour ; — but in the latter the casts are those of y>-eiAwa<er species, especially Cyclades. In 

 the subjoined sectional list, it will be found that clay occurs in detached portions above the Port- 

 land stone, in the great quarry at Swindon ; but I was not enabled to determine the formation to 

 which it ought to be referred. 



Portland stone. — The upper part of this formation at Dayhouse farm, east of the Reservoir, and 

 at Broom farm on the west of that place (Section No. 17.), abounds in the characteristic fossils, 

 and is immediately subjacent to ferruginous sand. In many of the quarries here, especially 

 in the east of the town, even the uppermost beds contain a large proportion of sand ; pre- 

 senting, in this respect, a striking contrast with the top of the series on the south-east coast, 

 but much resembling that of the Lower Boulonnois, especially of the quarries at Mont Lambert. 

 The stone is generally dispersed throughout this sand, in rugged and irregular concretional 

 masses, which, as well as the sand, contain numerous obscure remains of shells, frequently in 

 the stale of a calcareous powder. 



The following is a sectional list of the strata in the great quarry on the south-west of Swindon, 

 which occupies a space of no less than fourteen acres ; the beds rising from a point a little east 

 of south, at an angle between 1° and 5°*. 



Section of the Great Quarry at Swindon. 



Ft. In. 



\. Soil 6 



2. In some places, immediately below the surface is clay, occasionally 2| feet thick, with "i 



a slight covering of ferruginous (Lower green) sand. But in several places the thick- >■ 

 ness of this sand was as much as 4 feet J 



3. Loose rubbly stone 3 



4. A group, as follows : 



a. Limestone full of petrifactions. Cardium dissimile ; Cythereal rugosa, PI. XXII. 



fig. 13.; Gastrochcenal; Lucina Portlajidica,F\.XXll. fig. 12.; Pernaquadrala; 

 Pleurotomaria; Terebra Portlandica, PI. XXIII. fig. 6. ; Trigonia gibbosa; T. in- 

 curva; Long bones of Saurians \i 



b. Bluish grey limestone 



c. Clay 



d. Soft limestone, abounding in casts of shells J 



of the Wealden group was unknown ; and that in point of fact, the Portland stone is found in many 

 parts of the counties above mentioned, at the bottom of the Lower green-sand, and in a group 

 which is itself in part composed of sand : — so that where the latter sand is not well distinguished, 

 as in the section represented in the woodcut on the last page, the whole might easily be mistaken for 

 one continuous stratum. What is stated above, explains, I believe, the true relations of these 

 sands, to each other and to the Portland stone. 



* A mill on the north of the hill was deprived of water in consequence of the springs which 

 appear at the junction of the Portland sand with the clay, having been opened on the south-west 

 side of it, — ^so as to tap, as it were, the summit, and draw off the whole of the water ; the reten- 

 tive surface of the clay being inclined towards the south. This fact proves also the continuity of 

 the beds which form the top of the hill, and the absence of fissures — of such depth, at least; 

 as to penetrate the clay. 



