216 P1?0F. s. H. EEYXOLDS OX THE LITHOLO&ICAL vol. Ixxvil, 



ir. Desceiptiox or the Rocks. 



(A) Tiie Cltistopora Beds. (Thicki]ess = al)Out 468 feet.) 



(1) The Jlodiola Shales, Km. ( Thickness = about S8 feet.) 

 Extent, from the base of the section to the ' Biyozoa-Bed." 

 There is a double section of these rocks, one in the upper, one in 

 the lower, Avonmouth railwav-line. The latter is the iDetter one, 

 and will be here described. The rocks are a transitional series 

 from the Old Red Sandstone, and were clearly accumulated under 

 shallow-water conditions, as is shown bv I'i) the character of the 

 organisms, (ii) the frecjuent rounding of the fragments by rolling, 

 and (iii) the occurrence of penecontemporaneons brecciation (des- 

 sication-breccia). The actual passage from the Old Red Sandstone 

 t(3 the Carboniferous is bricked up. 



(a) The Lower Beds. — The preA'alent lithological type of 

 the lower part of the Jlodiola Shales is sandy, often calcareous 

 shale, passing into calcareous grit or gritty limestone, according 

 to whether siliceous or calcareous matter predominates. There 

 are also near the base of the section several bands of gritty argil- 

 laceous rock wherein the lime is aggregated mto nodules, which 

 sections show to contain the probably algal structure described by 

 Griiriehas Sj^ongiostroma (Al, A3). Other rocks consist of sandy 

 and argillaceous material crowded with ostracods, alternating with 

 narrow calcareous bands full of the alga Jlitclieldeania. (PI. X, 

 fig. 1) sometimes associated with OrfoneUa. These algal layers 

 (see PL Till, fig. 2) sometimes shoAv brecciation. Bryozoa are 

 present in some sections (A 10. 11. 12 j. Solenopora (PL X. 

 fisr. 2), thouo-h found by Prof. E. J. Oarwood^ and occasionallv 

 present in the sections exam.ined. does not appear to be common. 



Detinite bands of limestone several inches thick are also met 

 with as one ascends the series, and while in most cases crinoids are 

 the predominant components, in others brachiopods are the lime- 

 stone builders. Sometimes the small shell, crinoidal, and other 

 fragments are thoroughly rounded, perhaps in part by solution, in 

 part by rolling (All, PL X, fig. 3). Some bands of compact 

 black limestone are, however, chiefly composed of ostracods. some- 

 times associated (Al, Ao) with small Spirorbid annelids, gastropods, 

 and calcareous algce. Small gastropods are also a characteristic 

 feature of certain levels in the Cleisfopora Beds of Burrington and 

 Portishead. Miss M. B. Chapman,- who gives an analysis of a 

 limestone from this level, records the presence of sponge-spicules. 

 Ostracods are ^Dlentiful in ahnost all the rocks, whatever their 

 lithological character. 



^ Eep. Brit. Assoc. fBirmingliam) 1913, p. 466. The statement as to its 

 occurrence, in tte abstract of a paper read by th.e present anthor at the 

 Bonruemoutb. meeting of the British Association for the Advancement of 

 Science (Eep. Brit. Assoc. 1919, p. 183, and Geol. 3Iag. 1919, p. .523). requires 

 modification. 



- G-eol. Mag. 1912, p. 499. 



