MIDDLE LIAS. 187 
The total thickness of the Middle Lias in Dorsetshire is 
345 feet ; in Somersetshire, from a few feet to 230 feet ; in 
Gloucestershire, it is from 60 to about 280 feet ; in Northampton- 
shire (Kettering) and in Lincolnshire (Grant ham) 150 feet. 
Throughout the Jurassic Series there is probably no more 
definite plane of separation than that between the Middle and 
Upper Lias, for the so-called Transition-bed is so thin that it 
nowhere interferes with the boundary-line. The basement-beds 
of the Upper Lias are pale, earthy, and compact limestones, that 
occur in a more or less interrupted or ncdular form, in clays, and 
they yield Ammonites bifrons, A. communis, A. serpcntinus or A. 
falcifer, and other species in fair abundance, although some of 
them occur sparingly in the Marlstone. Lithologically, palaeonto- 
logically, and stratigraphically there is rarely any difficulty in 
deciding the junction, except in a few localities where the Rock 
Bed of the Middle Lias is not developed. 
While the Upper Lias is generally to be distinguished from the 
Middle Lias, it is a matter of great difficulty to fix a divisional- 
plane between the Middle and Lower Lias. This arises from the 
fact that the division has been taken in a series of marls and 
clays, it is established entirely on pala3ontological grounds, and 
authorities differ on the question where it should be taken. 
It matters but little where the division is taken under such 
circumstances, so long as the same approximate stratigraphical and 
palaeontological horizon is adopted. On the Geological Survey 
Maps the line of demarcation has generally been taken at the base 
of the zone of Ammonites margaritatus although exceptions occur, 
inasmuch as near Banbury, the blue clays containing this Am- 
monite have been included with the Lower Lias. Such incon- 
sistencies are likely to arise when the mapping depends on 
pala3ontological evidence, for this evidence is not always forth- 
coming. Prof. Green in discussing this question, has suggested 
that it would be better on the Geological Maps to survey simply 
on lithological grounds separating the Ciays, Sandstones, Iron- 
stones, and Limestones, and " leaving it to the paleontologist to 
decide to which of the three subdivisions the strata distinguished 
on the map ought in each locality to be assigned."* 
Such a plan undoubtedly would best serve economic purposes, 
and it might, perhaps, be adopted without neglecting at the same 
time to indicate the strata that are on the same stratigraphical 
horizon ; for we have seen that the development of limestones in 
the lower portion of the Lower Lias is subject to much variation, 
and that by attention to the foesils it is possible to mark 
approximately the different stages of the formation. In the same 
way an approximate boundary (for there is no real boundary), 
can be drawn between Middle and Lower Lia>s by separating the 
deposits in which here and there Ammonites capricornus and A. 
Henlcyi may be found in fair abundance, from those yielding 
A. margaritatus. 
* Review of "The Yorkshire Lias," Nature, December 7, 1876, p. 114. 
