431 
CHAPTER XIV. 
GREAT OOLITE SERIES. 
CORNBRASH. 
GENERAL ACCOUNT OF THE STRATA. 
CORNBRASH is :m old agricultural term, npplied in Wiltshire to 
certain stony or brashy soils that are well suited to the growth of 
corn. These soils are derived from the strata that lie between 
the Forest Marble and Oxford Clay; and the name Cornbrash, 
adopted in 1812 by William Smith as a geological term for these 
strata, was published by Townsend in 1813. 
The Cornbrash, where exposed in quarries, usually presents a 
very rubbly appearance ; and the broken stone is often coated 
with tufaceous material. It consists-of tough irregular Inyers of 
earthy and shelly limestone, alternating with softer beds of pasty 
limestone, and with partings and bands of marly or sandy clay : 
occasionally ihe limestone is sandy. The be-Js are usually of a 
buff or pale-grey colour, but the lower layers are frequently 
blue-hearted; and where the strata are protected from the 
weather by a covering of clay, the stone is for the most part of 
a bluish-grey colour, and it occurs in beds sufficiently solid to 
be employed locally for building-purposes. The rock hardly ever 
exhibits any traces of oolitic structure; and no indications of 
false-bedding have been observed. Occasionally we find sandy 
and gritty layers. 
The formation is usually from 10 to 25 feet thick ; sometimes 
even leys, and in places near Bedford it is represented by a single 
layer of rock. Nevertheless in lithological characters, and in the 
fossils which usually are to be found in abundance, the formation 
is remarkably uniform in its development throughout the country. 
In most cases it is clearly separated from the clays and flaggy 
limestones of the Forest Marble on which it rests in the south of 
England ; and further north where resting on the Great Oolite 
Clays, the junction as a rule is equally nun ked. In north Lin- 
colnshire however the Great Oolite Limestone presents characters 
resembling those of the Cornbrash and care is needed to distinguish 
them. In this region and in Yorkshire, the Cornbrash is over- 
laid by clays that yield Avicula echinata, and these have been 
termed the Cornbrash Clays.* Whether they represent in time 
a portion of the Cornbrash of the south of England, or a portion 
of the Kellaways Clay, is a subject on which no definite opinion can 
be expressed. As will be seen, there is no palaeontological break 
in the south of England nor elsewhere, between the Cornbrash 
and the Oxfordian series, for in the Kellaways Rock we find more 
or less abundantly some of the characteristic fossils of the Corn- 
brash. Hence we are nowhere justified in fixing time-limits by 
* Fox Strangways, Jurassic Rocks of Yorkshire, vcli i. p. 263. 
