Geological and Agricultural Divisions. 279 
two porous strata, by the one leaving off when the other begins, 
form a tortuous belt which extends from Thame in a north-west 
direction for a distance of almost 30 miles. All the beds of this 
oolite, which is a loose granular limestone, are not of any great 
thickness, and have evidently three well-marked divisions. On 
the top is a soft white stone resembling chalk, which is chiefly 
used for lime ; the next layer produces good building-stone ; 
while the bed below contains numerous casts of shells which 
so injure the stone that it is used for coarser purposes. How- 
ever, these divisions are not always in the above order ; the 
second vein of stone at Brill is highly contorted, and is used for 
pitching ; it hai-dens by exposure to the air, while the bottom 
layer is best adapted for building. The colour of the stone, which 
varies much, is due to the presence of iron. On all the insulated 
hills which are spread over the vale of Aylesbury, the Portland 
oolite is found near the summit. It rises through Long Crendon 
and Chilton, and culminates at Brill, where it attains its greatest 
elevation of 744 feet. In the elevated ground about Winchenden 
and Waddesden, and again at Whitchurch and North Marston, to a 
mile beyond Stewkley, it appears in detached masses. Not that the 
rock is ever of considerable thickness : the only spot of any great 
solidity is at Creslow, where it is 8 feet thick. On this the 
house is built, and the cellar, formerly the crypt, is hewn out 
of the solid rock. Whenever the stone appears in these broken 
portions it seldom presents any breadth of surface, being more 
frequently covered with other deposits. Thus at Crendon the 
stone is overlaid with a thin band of ferruginous stone, covered 
with a seam of the greensand, containing minute particles of 
ochre and finely granulated pebbles. Above this again is a 
deposit of the gault, which forms a stiff clay on the summit of 
the hills, and has a few rolled flints mixed in the upper soil. 
These flints having been transported and exposed for ages to the 
influence of the air and the percolation of water, are altered 
from black to brown, yellow, and red, which has led some to 
suppose them to be another sort of stone. The principal deve- 
lopment of the Portland oolite, Avhich commences at Scotsgrove, 
forms a tract of exceedingly useful arable land. It is a dry 
sandy loam, in some places light, in others more retentive, and 
very fertile — all capable of yielding excellent crops with good 
management. There are on it a few patches of peat, which vary 
from a few inches to several feet in thickness. At Stone, just 
above the rock, there is a good depth of pure sand which changes 
from yellow to quite white. It is a disputed point among 
geologists if this is a Wcalden or an oolitic sand ; however, it 
matters not on agricultural grounds, as it only crops out in small 
patches. The attempt to sink an Artesian well at the County 
