248 Preston: Geology South of Grantham. 
these sands are very ferruginous, being, in fact, a good iron- 
stone, whilst in a well sunk at Harrowby to the N.E. o 
Grantham, a pure white quartzose bed of sand about two feet 
thick was passed through just before reaching the Lias Clay. 
I may here remind you that the Jurassic System consists of a 
great series of fossiliferous rocks, composed of variable beds of 
clays, sandstones, and limestones, and which extend across 
England from the Yorkshire coast on the N.E. to the Dorset- 
shire coast on the S.W. e lower division of this great and 
i 
moderately deep water conditions, and which occupied the site 
of what was previously great inland salt lakes and barren 
desert lands. The total thickness of these clays varies locally 
from 800 feet to 1,000 feet, and there is a great and gradual 
change in species between the bottom and top beds, which in 
itself indicates a vast period of time during which the deposits 
were accumulated. Our study to-day will lead us only to the 
top member of this formation, the Upper Lias, a series of clays 
about 120 feet thick. Locally they may be divided into four 
zones, the lines of division not being very clearly defined. 
The highest beds of the Upper Lias are characterised by the 
abundance of a small bivalve shell called Zeda ovum, and hence 
called the Leda ovum beds. These beds used to be well seen in 
a small pit at Colsterworth, and the pit has yielded, in addition 
to z ovum, Ammonites bifrons, Belemnites, and Myacites 
donaciformis rather abundantly, whilst several specimens of the 
Brachiopod shell Dzsczna reflexa have been found attached to 
the shells of Leda ovum. Am. heterophyllus and communis also 
occur very sparingly. 
Below the Leda ovum beds we have the Comets ada: the 
upper 50 feet of which is not very fossiliferous. The base of the 
_ Communis beds consists of clays crowded with small Ammonites, 
_ chiefly Am. communis and Am. annulatus. These beds have been 
well displayed in Rudd’s brickyard on the south side of the 
- town, and numerous fossils have been collected. The unfossili- 
ferous portion of these beds is seen in the Brick Company’s yard 
in Papermill Lane and in the Spittlegate railway cutting, and is 
characterised by the common occurrence of crystals of selenite, 
pyrites, and jet, and are termed the Selenite Clays. Below the 
Communis beds occur the third division called the Serpentinus 
oe beds, clays with large nodular concretions, and in which > 
Repeal of this | Se are dateijetnane d abundant. 
i 
“Naturalist, ‘ 
