X. 



SMITH'S CLASSIFICATION. 



113 



LOWER LIAS. 



For a general view, in the midland districts, it is often found 

 useful to adopt a binary division of this thick series, founded on the 

 prevalence of shale in the upper, and of limestone beds in the lower 

 part of the series. The lower portion was again divided by Smith 

 into blue lias above, and white lias beneath ; the latter containing 

 few fossils; the distinction being founded on observations in the 

 vicinity of Bath. His white lias was typified about Radstock ; his 

 blue lias about Keynsham. 



A capital railway section at Saltford, carefully measured by Mr. 

 W. Sanders, affords probably the best view of the blue and white 

 lias in connexion with the Westbury beds which is known in the 

 region of Bath and Bristol. 



In the Saltford section about no feet of alternating limestones 

 and shales occur, all belonging to the lower part of the lias ; the 

 lowest part of all corresponding, but not very accurately, to the 

 Westbury beds. 



They may be thus summarized : 



ft. 



f. Beds of limestone and shale containing Belemnites acutus . . .15 

 e. Beds of limestone and shale containing Ammonites Bucklandi . . 10 

 d. Beds of limestone and shale containing Gryphsea incurva . . .20 

 c. Beds of thin flaggy limestone and shale containing few fossils * . 

 b. The lower portion, regarded as white lias, rests on a peculiar nodular bed > 40 



called Gotham marble . J 



a. Series regarded as equivalent to the Westbury shales, but much more 



calcareous 25 



Pleurotomaria similis occurs in e. 

 Lima gigantea occurs^ c, d, e. 

 Pholadomya glabra occurs in c, d, e. 

 Spirifera Walcotti occurs in e. 



Ammonites Conybeari occurs in d, e, f. 

 Ehynchonella variabilis occurs in d, e, 



f. 

 Ichthyosaurus occurs in d, e. 



The fine fossil formerly known as Plagiostoma (now Lima) gi- 

 gantea occurs more or less frequently in all the beds of this section 

 which contain ammonites. The whole group of strata has been for 

 this reason not inconveniently called the ' Lima 3 leds. 



* At Lyme Regis this part of the section yields two zones, one marked by A. an- 

 gulatus, the other, still lower, marked by A. planorbis. The lowest beds contain no 

 ammonites or belemnites. 



