22 JJAS OF ENGLAND AND W.\Ll>> 
We have thus a great group of strata that lie above the White 
Lias and its equivalents (belonging to the Rhsetic Beds), and 
below the oolitic limestones and associated strata of the Inferior 
Oolite Series. In mass it is an argillaceous series, consisting o 
bluish-grey shales, clays, and marls, with layers of argillaceous 
limestone and nodular masses of limestone (cement-stones and 
septaria), with thick beds of sand and calcareous sandstone, 
ferruginous earthy and sometimes oolitic ironstone and limestone 
(marlstone). 
The Lias attains a thickness of about 900 feet in Dorsetshire, 
decreasing to about 280 feet at Bath, and becoming more 
attenuated under Oxford. It expands again to 1,360 feet, in North 
Gloucestershire, 760 feet at Northampton, 800 feet in Rutland, and 
about 950 feet in parts of Lincolnshire. In Gloucestershire we 
find a complete and unbroken series, and, with the exception 
perhaps of the Upper Lias, the same is the case in Dorsetshire ; 
but in parts of other counties the sequence is to some extent 
interrupted, there being symptoms of a break between the Upper 
Lias Clay and overlying Oolitic strata. 
On the whole the general characters of the beds are fairly 
persistent, although in places Avhere the Lias fringes the Palaeozoic 
rocks, some striking modifications are met with, and the strata 
become much attenuated. The rock-beds of the Middle Lias 
are the most variable in the series. 
The relations of the Lias to the Rhsetic Beds below and to the 
Inferior Oolite Series above will be discussed further on. It 
may however be stated that the Lias and Rhaetic strata are 
usually conformable, and over large areas the Lias and Inferior 
Oolite exhibit evidence of such gradual passage that no hard line 
of division can be fixed, and consequently opinions vary on the 
question of i_n*ouping particular layers. These passage-beds 
between the Lias and Inferior Oolite, known as Midford Sands, 
form a convenient stratigraphical division that is of local 
importance ; but it may be freely admitted that from a palseonto- 
logical point of view these Sands include portions of both Upper 
Lias and Inferior Oolite, the equivalents of which elsewhere may 
more definitely be separated. 
The general character of the changes met with in the Liassic 
formation are shown in the accompanying diagram. The vertical 
scale being so much in excess of the horizontal, an unnatural 
appearance of disturbance is produced in the vicinity of Bath 
where the beds are much reduced in thickness. The object of 
the diagram is to picture tho main changes that the strata 
undergo ; and it will be seen that the irregular development of 
the Lower Lias limestones, and the persistent nature of the 
junction between Middle and Upper Lias, are the more striking 
features. (See Fig. 1.) 
With regard to the former extent of the Lias, we have evidence 
of the margin of the old sea at certain points on the M ndips 
and in Glamorganshire, but it is probable that these land-areas 
were only islets. We know not how far the Lias extended over 
