229 



and I have long believed that these two thin bauds, Nos. 17 and 18, ax-e 

 the equivalents of the Gotham marble, at Saltford, Gotham, Patchway, 

 and some other places. But more esjiecially to define the horizon of the 

 Monotis and Modiola series here, I believe we may pai-allel the four 

 beds, Nos. 17, 18, 19, and 20, with the White Lias series; for, thin as 

 they are, yet from observations made at other sections, I have 

 confidence in assigning to them the position, and that they are 

 the representatives of what is now regarded as the White Lias 

 of typical localities. Again, the lower or Myacites band, and 

 the Monotis Limestones have every element in them, both as 

 regards position, petrological structure, and organic contents, under the 

 modifying agencies due to alternations of level and other physical con- 

 ditions, to induce us to parallel them in time with the White Lias, and 

 which are succeeded here, as elsewhere, by the true Ostrea liassica beds 

 above ; these beds, Nos. 15 to 20, are of great interest, and we have yet 

 to learn much respecting them, through correlation with other sections, 

 especially those of Aust Pa.ssage, and Patchway on the Bristol and South 

 Wales Railway, or Kew Passage Line, where these grey and white Lime- 

 stones* for many miles determine the spread of the Rhsetic scries, and the 

 uneven or eroded surface of the Gotham marble at Patchway, although 

 only three inches thick, faithfully chronicles through its conservative 

 character, and the persistency of similar conditions at the close of the 

 black Shales and marls, at Westbury section, how it has resisted the 

 powerful effects of denudation, and presei-^'ed to us from destrviction the 

 soft and pyritous shales below.* 



The species found in these two thin but important bands, ^^z., the 

 Monotis and Myacites beds, Nos. 17 and 18, are the following: — 



Myacites musculoides, Schl. 



Modiola minima, Soiv. 



Auatina 



Monotis (Avicula) decussata Goklj'. 



Gai-dium Rhseticum, Merian. 



Ostrsea liassica, Sirickl. 



Libellula, wing of 



* The Grey Marls and Argillaceous Limestones at Aiist Passage, attributed by 

 some to tlie Zone of A. Planorbis, belong instead to the horizon of the Monotis and 

 Myacites marls, at Westbury, and the " lowest Limestone therein mentioned is the 

 locally peculiar Cotham marble, which contains the Elytra of C'oleoptera, wings of 

 insects, and scales, and perfect fish of Legnonotus Cothamensis, Eg. , and Pholido- 

 phorus H'njginsi, Strickl. 

 p2 



