ZONE OF AMALTHEUS MARGARITATUS. 99 



aequivahis. Sow., with the highly characteristic P. dentatus, Sow., also P. sublavis, Phil., 

 Hinnites abjectus, Phil., and Avicula inaquivalvis. Sow., are among the most abundant 

 forms in the Rock-bed. 



"The Marlstonc 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 IS feet 6 inches in 



ithickness ; 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 junction 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 Marlstonc 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 Amaltheus maryaritatus, and e, the 

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

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

 describing the sjjinatus-hQ.A?, in Dorsetshire, wc shall find that the Ammonites of the upper 

 Lias, Stephanoceras commune and Stej)!/. annulaium, are sometimes found side by side with 

 Amaltheus spinatus, 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 Margaritatus-h^i^'s, and their fossils 

 from the different brick-yards examined, and of the Spinatus-hed 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 Marz/aritatus-heds 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 3Ia)yaritatus-heds of 

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



The Marlstone Rock-bed, or Ammonites spinatus zone forms the top of the Upper 



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



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, 18/2. 

 2 Ibid., p. 6. 



