ZONE OF AMALTHEUS MARGARITATUS. 99 



<eqiiivahns, Sow., with the highly characteristic P. dentatm. Sow., also P. sublavis, Phil., 

 Hinnif.es abjedus, Phil., and Avicula incequivalvis, Sow., are among the most abundant 

 forms in the Rock-bed. 



" The Marlstone Rock-bed is very variable, both in thickness and mineralogical 

 -character; it is finely developed in the neighbourhoods of Tilton-on-the-IIill and 

 Somerby, near the former of which places it is seen to measure 18 feet 6 inches in 



thickness ; towards the east and south, however, it attenuates very rapidly 



Besides being greatly diminished in thickness, the Rock-bed sometimes loses its cal- 

 careous character and becomes sandy When the jtmction of the Upper 



Lias clay and the Marlstone Rock-bed is seen, the latter often presents the appearance 

 of having suffered erosion before the deposition of the former." 



I have given my learned friend's description in extenso, as it is a very clear and 

 concise account of the Marlstone Rock-bed of the Middle Lias as it exists throughout the 

 midland district of England. The Lias a, h, c, d, according to the classification I have 

 followed in this work, would belong to the zone of Anialtheus maryaritatus, and e, the 

 Rock-bed, would fairly represent the zone of Amallheus spinatus. It is noteworthy in 

 passing that both in this section, and in another which I shall give of Down Cliff when 

 describing the spinal us-htAs, in Dorsetshire, we shall find that the Ammonites of the upper 

 Lias, StepJianoceras commune and Stej)!/. anmdaftim, are sometimes found side by side with 

 Amallheus spinaliis, which dies out in the upper part of this zone. 



I must refer the student for ample details about these two zones to Professor Judd's 

 memoir, as it is replete with instructive sections of the Marffaritatus-hed?, and their fossils 

 from the different brick-yards examined, and of the 8pinatus-he6. or Marlstone Rock- 

 bed at Robin-a- Tiptoes. 



In Warwickshire this zone is very well developed near Banbury, where it has been 

 long and carefully studied by Mr. Beesley, F.C.S., who has given a very good account of 

 the formation.^ According to this author the Marffaritatus-heA?, form the base of the 

 Upper Middle Lias, and consist of blue micaceous marly clay, with nodules of hard marl, 

 or claystone ; above this are hard bluish calcareous sandstone embedded in sand, and 

 overlain by sandy marl or clay, containing the usual fauna of the Margaritalus-\)&i\s, of 

 Rutland, with many additional species collected through several years by local geologists. 



The Marlstone Rock-bed, or Amnioniles sjyinalus zone forms the top of the Upper 

 Middle Lias, and is its most characteristic feature, being largely spread over the distiict, 

 and forming a broad table-land. On the west and south, and a terrace on the east side 

 of the valley, the disintegration of its friable stone has produced the rich red land so well 

 adapted for wheat-growing." The Rock-bed is a ferruginous often sandy limestone, 

 externally brown, but of a greenish-blue colour in the interior of the blocks, and is 

 usually separated by thin partings of sandy loam or clay into two or three beds, 



' ' Sketch of the Geology of the Neighbourhood of Banbury,' p. 5, 1S72. 

 2 Ibu!., p. 6. 



