448 MESOZOIC TIME REPTILIAN AGE. 



The Wealden is wholly of estuary or fresh-water origin ; the beds 

 consist of clays, sands, and, to a small extent, limestone. 



The prominent subdivisions of the Jurassic formation observed in England 

 (though not present alike in all its Jurassic regions) are the following, beginning 

 below : — 



I. Lias. 



1. Lower Lias: consisting of grayish laminated limestone, with shale 

 above, and a bone-bed and marls below. 



2. Middle Lias : a coarse shelly limestone called marlstone. 



3. Upper Lias : beds of clay or shale with some thin limestone layers. 

 II. Oolite. 



1. Lower or Bath Oolite, consisting of — 

 (1.) The inferior Oolite, a limestone with fossils and layers of sand. 

 (2.) Fuller' s-earth group, or clayey layers. 

 (3.) The Great Oolite, limestone mostly oolitic. 



(4.) Forest-marble group, sandy and clayey layers, with some oolite. 

 (5.) Cornbrash, a coarse shelly limestone. 



The Stonesjield slates, noted for their remains of Saurians, as well as of 

 the earliest British mammals, and also of insects and other species, occur 

 near Oxford in England, and belong to the Lower Oolite, below the 

 Great Oolite. 



At Brora, in Sutherlandshire, there is a bed of Oolitic coal of good quality, 

 three and a half feet thick, which has been long worked : it is covered 

 by several feet more of impure coal containing pyrites. It is supposed 

 to belong with the Great Oolite. 



2. Middle or Oxford Oolite : consisting of — 

 (1.) The Kelloway Rock, a calcareous grit, overlying blue clay, 



and overlaid by the Oxford clay. 

 (2.) Calcareous grit and oolitic coral limestone, called the Coral 

 Rag. 



3. Upper or Portland Oolite : consisting of — 

 (1.) Kimmeridge Clay. 



(2.) Shotover Sand, a calcareous rock with concretions. 

 (3.) The Portland Oolite. 



4. Purbeck beds : consisting of (1) the Lower Purbeck, fresh-water marls 

 with the " Portland dirt-bed," and resting on the upper layers of the 

 Portland stone ; (2) the Middle Purbeck, mostly a bed of marine lime- 

 stone, 30 feet thick ; (3) the Upper Purbeck, 50 feet of fresh-water depo- 

 sits. The dirt-bed of the Purbeck is the second deposit affording re- 

 mains of British mammals. It contains also numerous remains of 

 Cycads, etc. 



III. Wealden. 



1. Hastings Sands : sandstone with some clayey and limestone layers, 

 containing Saurian remains, fluviatile shells, etc. 



2. Weald Clay : clayey layers, with some calcareous beds - containing 

 fresh-water shells. 



The British subdivisions are for the most part recognized in France, and 

 have received special names by D'Orbigny. They are (I.) in the Lias, — 1, the 



