JURASSIC PERIOD. 435 



The promiment 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. 



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



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



II. OOLYTE. 



1. Lower or Bath Oblyte, consisting of — 



(1.) Jnferior Oblyte, a. limestone with fossils and layers of sand. 



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



(3.) Great Oblyte, limestone, mostly oclitic. 



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



(5.) Cornbrash, a coarse shelly limestone. 

 The Stonesfield 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 Oblyte, below the Great Oblyte. 



At Brora, in Sutherlandshire, there is a bed of Oolytic 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 pyrite. It is supposed to belong with the Great Oolyte. 



2. Middle or Oxford Oblyte: consisting of — 



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



by (2.) the Oxford clay. 

 (3.) Calcareous grit and oclitic coral limestone, called the Coral Rag. 



3. Upper or Portland Oblyte : consisting of — 



(1.) Kimmeridge Clay. 



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



(3.) Portland Oblyte. 



4. Purbech 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 limestone, 30 feet 

 thick; (3) the Upper Purbeck, 50 feet of fresh-water deposits. The dirt-bed 

 of the Purbeck is the second deposit affording remains 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 re- 

 ceived special names from D'Orbigny. They are (I.) in the Lias, — 1, the Sinemurian 

 (Lower Lias, named from the locality at Se'mur); 2, Liasian (Middle Lias); 3, Toarcian 

 (from the locality at Thours); (II.) in the Oolyte, — 1, Bajocian (the inferior part of 

 the Lower Oblyte, named from the locality at Bayeux); 2, Bathonian (the Great Oblyte, 

 Bath Oblyte): 3, Callovian (Kelloway Rock); 4, Oxfordian (Oxford Clay); 5, Corallian 

 (Coral Rag); G, Kimmeridgian (Kimmeridge Clay); 7, Portlandian (PortlandOblyte). 

 In the French Juras, the Lias limestone is called also Gryphite limestone, from the 

 abundance of the fossil Gryphaia incurva. 



For the '"Inferior Oblyte" Marcou has used the name Lozdonian; for the Fuller's 

 earth, Vesulian. Thurman and Etallon have restricted Corallian to the lower part of 

 the Corallian of D'Orbigny (the part called Rauracian by Creppin), and named the 

 upper part, commencing with the beds containing Astarte minima and including the 

 lower part of the Kimmeridge clay, the Astartian (the same is the Sequanhn of M. 

 Jourdy); the Kimmeridgian, comprising the middle part of the Kimmeridge Clay, is 

 the Sfrombian of Thurmann. The Portland Oblyte is the Portlandian of Marcou (or 

 Virgulian of Thurmann) ; and, lastly, the Purbeckian is the Dublisian of Desor and 

 Tithonic of Oppel. The "YVealden is Lower Neocomian of D'Orbigny. 



