MICROSCOPIC STRUCTURE. 33 
Sections of Liassic limestone have been examined under the 
microscope by Mr. J. J. H. Teall, whose notes are embodied in 
the following statement : 
The fine-grained and more or less earthy limestones of the Lower Lias, of 
which examples were examined, from Street and Shepton Mallet in Somerset- 
shire, and from \Vigston in Leicestershire, show in section granular calcareous 
matter. Organic fragments 'belonging to Crinoids and Echini may be noticed, 
and these often appear in a matrix of crystalline calcite, the fragments some- 
times forming nuclei of large individual fragments of calcite, as in the Doulting 
stone (Inferior Oolite). 
The granular and shelly limestones of the Lower Lias of Sutton in Gla- 
morganshire, of Shepton Mallet and Downside near Wrington in Somersetshire, 
show a matrix of tine-grained semi-transparent and partially crystalline cal- 
careous matter, with obscure and more or less rolled organic fragments. 
The ironshot limestone (Marlstone) of the Middle Lias of Trent, near 
Yeovil, showed many organic fragments and a few grains cf ferruginous 
oolite. The matrix was composed of calcite, partly granular and partly 
crystalline ; secondary caicite often occurring in optical continuity with the 
original organic fragments. 
The green earthy limestone (Marlstone) of Hornton, near Edge Hill, in 
Warwickshire, showed rolled organic fragments in a matrix of crystalline 
calcite. 
An argillaceous limestone from the Upper Lias of Brent Knoll in Somerset- 
shire, showed organic fragments (Foraminifera, &c.) in a dull grey granular 
matrix. 
The Lias clays seem to be analogous to the blue terrigenous 
muds described by Messrs. Murray and Renard : such deposits 
are extensively formed around the great continents and continental 
islands, and they are coloured blue by organic matter and by 
sulphide of iron.* 
Subdivisions of the Lias. 
The principal classifications of the Lias adopted by different 
authors having been tabulated by Mr. Strangways,t it is un- 
necessary to repeat them here ; but it will be useful to state in 
concise form the grouping adopted in this Memoir. 
On pakeontological grounds it is indeed difficult to separate 
the Middle and Lower Lias, and different authorities have adopted 
different zones for the upper limit of the Lower Lias. Thus 
Wright, following Oppel, took the zone of A. raricostatus as the 
top of the Lower Lias, and the same horizon was followed by 
Tate and Blake.J Tate however admitted that the line of 
demarcation might be drawn between the zones of A. obtusus and 
A. oxynotus. Prof. Judd. following Quenstedt, took the junction 
between the zones of A. capricornus and A. margaritatus, a course 
that on stratigraphical grounds is the most convenient, although 
not unattended with difficulty, and it is one also that coincides 
most nearly with the original divisions made in the Lias by John 
Phillips. || This grouping is f3llowed in tire present Memoir. 
* Report of the " Challenger," vol. i., Part 2, 1885. 
f Jurassic Rocks of Yorkshire, vol. i. p. 29. 
J Yorkshire Lias, p. 16. A. de Lapparent in his Traite de Geologic (1893), 
p. 953, follows the same grouping of the Middle Lias as Oppel ; and employs the 
name Ckarmouthian for the zones A. armatus to A. spinatits (inclusive). 
Quart. Journ. Geol. Soc., vol. xxvi. p. 400. 
j| Geol. Kutland, p. 46. 
E 70859. C 
