342 PHYSICAL AND PAL&ONTOLOGICAL BREAKS. 



centages of species which pass between the divisions of the Lias are 

 remarkable. Thus about one-third of the species pass from the top 

 zone of the Lower Lias into the Middle Lias ; and from the top of 

 the Middle Lias only about 5 per cent, pass into the Upper Lias ; 

 while from the Upper Lias 27 per cent, pass into the Upper Lias 

 sands, and 2 1 per cent, into the Inferior Oolite. One hundred species 

 are recorded from the Rhsetic beds, 281 genera and 1830 species 

 occur in the Lias, of which 1080 are in the Lower Lias, 562 in the 

 Middle Lias, and 418 in the Upper Lias. Forty-five species survive 

 to the Inferior Oolite, and 1 1 to the Great Oolite ; about half in both 

 cases being bivalves. 



There is no complete physical break between the Inferior Oolite 

 and the Lias, though the mineral character of the bed alters, and 

 between Yorkshire and Gloucestershire this might be considered to 

 amount to an unconformity. The Inferior Oolite contains 1000 

 species, of which 65 extend into the Fuller's Earth, and 175 into 

 the Great Oolite. The Fuller's Earth thins out entirely to the N.E. 

 of Cheltenham. It is only known to contain no species, of which 

 65 are common to the Inferior Oolite, and the same number range up 

 to the Great Oolite. 



The Great Oolite contains 820 species, of which 84 range to the 

 Forest Marble and 120 to the Cornbrash. The Forest Marble con- 

 tains 136 species, of which 48 range up to the Cornbrash. Between 

 Yorkshire and Dorsetshire all these lower oolites may be regarded 

 as unconformable to each other, though no actual unconformity is 

 seen. 



The Cornbrash may be regarded as forming a break with the un- 

 derlying strata, since it is the only deposit of the Lower Oolites which 

 ranges through England from Dorsetshire to Yorkshire. It contains 

 244 species, of which 56 range up into the Kellaway Rock, and 48 

 range down to the Forest Marble. The Kellaway Rock has 168 

 species, of which 60 pass into the Oxford Clay. The Oxford Clay 

 contains 73 genera and 194 species, of which 48 pass up into the 

 Coralline Oolite and 25 into the Kimmeridge clay. Still there is no vis- 

 ible physical break in the succession in the British area. The Corallian 

 fauna, which abounds in lamellibranchs, gasteropods, echinoderms, and 

 ammonites, comprises 116 genera and 334 species, of which only 14 

 are corals. The Kimmeridge Clay only receives 33 species from the 

 Coralline Oolite, and of the 263 species found in the bed, only 22 

 survive into the Portland Oolite. The Portland Oolite, greatly 

 limited in its area, has a fauna of 128 species, of which none survive 

 to newer rocks. 



The Purbeck beds pass so insensibly into the underlying Portland, 

 that the difference is only to be detected by the fossils ; but since the 

 Portland beds are marine, and the Purbeck largely fresh water, an un- 

 conformity must exist. Similarly an unconformity must be inferred 

 for the overlying Wealden beds, the distribution of which is different 

 from the Purbeck beds ; but there is no trace of an unconformity 

 between the Wealden and the Lower Greensand till we pass west and 



