245 
CHAPTER IX. 
UPPER, LIAS. 
GENERAL DESCRIPTION. 
THIS formation consists for the most part of bluish-grey clay 
and shale, with nodules of argillaceous limestone (cement-stones 
and septa ria), and occasionally a good deal of selenite. The Base- 
ment Beds consist of pale earthy limestones and marls or clays, 
the bands^of stone occurring sometimes in regular courses, but 
being often of an interrupted and nodular character ; while the 
nodules enclose remains of Fishes and other organic remains. 
These lower beds are usually opened up in quarries where the 
Marlstone is worked, and the junction is well-marked throughout 
the district, from the Dorset coast, where the Marlstone and Upper 
Lias stone are represented in a band about 2 feet thick, through 
the Midland counties to Lincolnshire. Stratigraphically this^is 
one of the most definite boundaries in the series of Jurassic rocks, 
for lithologically, palaeontologically, and in the sequence of strata 
there is rarely any difficulty in fixing the junction. The pale 
earthy limestones of these Basement-beds, resembling, as they 
often do in texture, beds of White Lias or of Great Oolite lime- 
stone, appear in marked contrast with the brown iron-shot or ferru- 
ginous Marlstone. In a few localities, however, where the Marl- 
stone is thin, or has become much decomposed near the surface, 
the boundary across country cannot be traced without considerable 
difficulty. 
Nevertheless there is no palseontological break between these 
formations. In Northamptonshire there is a thin layer known as 
the " Transition Bed," which by its fossils, links, the formations 
together ; and generally throughout the country, we find common 
to both Marlstone and Upper Lias, such forms as Ammonites 
annulatus, A. communis, A. crassus, A. Holandrei, A. serpentinus, 
&c.* There is no authentic record, however, of the occurrence 
of A. spinatus above the horizon of the Marlstone or Transition 
Bed. 
The thickness of the Upper Lias varies considerably in different 
parts of the country, and especially when we lake into account 
the sandy and clayey beds that belong to the zone of Ammonites 
jurensis. Excluding, for the sake of convenience, the sandy beds, 
and taking only the mnss of clays, together with the Basement 
Beds, we find the thickness of the Upper Lias in Dorsetshire to be 
about 70 feet, in Gloucestershire 100 to 200 feet, in Oxfordshire 
30 to 100 feet, in Northamptonshire 150 to 160 feet, in Rutland- 
* Day, Quart. Journ. Geol. Soc., vol. xix. p. 295 ; Ju<W, Geol. Rutland, p. 46 
K. Wilson and W. T). Crick, Geol. Ma<r., 1889, p. 341, 
