8 MK. L. RICHAEDSON ON THE RHJSTIC OF [Feb. I9II, 



abound, literally covering the surfaces of the thin pieces of limestone 

 that can be there split off from the bed. 



The main point to notice, however, so far as Somerset is con- 

 cerned, is that the Cotham Beds are persistent south of the Mendip 

 Hills, if not quite so thick as in North- West Gloucestershire, and 

 divide the Black Shales (Westbury Beds) from the White Lias 

 proper (Langport Beds). 



Langport Beds. — The change from Cotham Beds to Langport 

 Beds is considerable, palaeontologically more than lithically. In 

 other words, some pause in the normal sequence of deposit is again 

 suggested. The frequently Lithojohagus-bored. and Dimyodon- 

 enerusted nodules of Cotham Marble, as at Dundon Hill (p. 39); 

 the ' false ' or conglomeratic Cotham Marble of Culverhole, Somer- 

 ton, Sedbury Cliff, Aust Cliff, etc. ; the remarkable relationship of 

 the Cotham-Marble equivalent to the Langport Beds at Lilstock, 

 are all facts which point to a temporary interruption of sequential 

 deposition, connected doubtless with the causes that brought about 

 the conditions suitable for the formation of the richly-fossiliferous 

 Langport Beds or the well-known Wbite Lias proper. 



In Somerset, in the fine section in the railway- cutting at Charl- 

 ton Mackrell, where the Langport Beds were so magnificently 

 exposed, they were separable into five groups. The first and top 

 one embraced the ' Sun '- and ' Block-Beds ' ; the second, the 

 ' Rubbly Beds ' and three massive limestones ; the third, more 

 regularly-bedded limestones ; the fourth, similar beds, but with a 

 massive top-bed, often coralliferous (Thecosmilia ? michelini Terq. 

 & Piette) : and the fifth, very massive limestones, locally called 

 ' the Sizes.' The mammillated and often bored upper surfaces of 

 the Sizes, together with their irregular nether surfaces ; the irre- 

 gular, rubbly, and conchoidal fracturing of many of the other beds; 

 and the way in which the fossils are attached to the upper surfaces 

 of the limestones or are embedded in the marl, all bear witness to 

 the slow rate at which the Langport Beds were formed. 



Dimyodon intus-striatus, which makes its first appearance in the 

 Pteria-contorta Shales and is usually absent from the Cotham Beds, 

 appears in great force. Moreover, specimens of Volsella minima 

 (Moore non Sow.), Ostrea liassica, Plagiostoma valonicnse, and in 

 places internal casts of Cardinia and Protocardia abound, but do 

 not appear to be of any use zonally. 



Watchet Beds. — These beds have a very restricted geographical 

 extent : it is approximately coextensive with that over which the 

 Sully Beds were laid down. They are not very interesting, com- 

 prising marls with inconspicuous and impure limestone-layers, that 

 contain occasional specimens of Ostrea liassica Strickland and 

 Volsella minima (Moore non Sow.). They are distinct from the 

 Langport Beds, decreasing in thickness eastwards as those increase, 

 and also from the Lower Lias that overlies them. 



