320 



The Geology of Devizes. 



visits them can only see that once upon a time an excellent and 

 interesting section was to be seen at the spot. 



I need not now say more about the Ferruginous Sands except to 

 express a hope that the time may come when the Ironstone they 

 contain may be utilised, and to point out that they were formed in 

 very shallow water along the shore of an ancient continent which 

 then lay to the west of Wiltshire. 



2. Gauli. Above the Ferruginous Sands, and forming the lower 

 part of the slopes below Roundway, Devizes, Potterne, Stert, and 

 Urchfont, is a dark grey clay known as the Gault. The brickyards 

 at Dunkirk, Caen Hill, Stert, and Lavington are opened in this 

 clay, and its thickness in this district is from 80ft. to 90ft. 



To the unassisted eye a lump of the clay only seems to be fine 

 dry mud glistening with minute particles of a silvery substance 

 which we know to be Mica. By the aid of the microscope we learn 

 that it chiefly consists of very fine mud, in which are scattered 

 small grains of Quartz, flakes of Mica, particles of Glauconite, 

 fragments of shell, and some perfect shells of the tiny creatures 

 known as Foraminifera. 



The lowest beds are well exposed at Caen Hill and Dunkirk 

 brickyards, the middle beds at Lavington, and the upper beds were 

 formerly worked on the slope at Dunkirk, but are not now exposed 

 at any place in the district. Here, again, we are indebted to Mr. 

 Cunnington for collecting from an exposure that proved to be 

 temporary ; for the fossils he obtained from this upper Dunkirk 

 brickyard are not the same as those which can be found in the lower 

 pit. At the latter the prevalent Ammonite is Am, interruptus, with 

 its variety Am. Benettia and occasionally Am. Budeanlii, while at 

 the higher pit Am. lautus, Am. tuberculatus, and Am. splendens were 

 found ; species which also characterise the upper part of the Lower 

 Gault of Folkestone. 



3. Malmstone. The upper part of the Gault becomes marly and 

 sandy, and passes up into a soft grey or buff sandy stone, which is 

 known as Malm or Malmstone. This is a peculiar rock, which has 

 not yet been found at any other geological horizon, but uniformly 

 occurs between the Gault and Greensand throughout the counties 



