SEQUENCE OF STRATA. 5 
and Spilsby Sandstone, and although the latter contains Port- 
landian fossils, yet these bear evidence of having been derived ; 
and the most that can be said on behalf of the Jurassic age of the 
Spilsby Sandstone, is that it may include representatives in time 
of the Purbeck Beds. 
The divisions into Upper, Middle, and Lower Oolites, adopted 
by the Geological Survey, are practically the same as those em- 
ployed by Conybeare in 1822; they are simply divisions that ure 
convenient for purposes of local description. 
In upward sequence we have to deal therefore with deposits 
that represent an alternating series o sedimentary conditions ; 
and although a comparatively small part of each formation is 
opened to view, yet the general characters of the chief sub- 
divisions are maintained along the line of strike northwards from 
Dorsetshire to the northern end of the Cotteswolds. The general 
strike turns in an easterly direction from the Cotteswold region to 
Northampton, and there we find evidence of the more important 
changes in the nature of the calcareous and arenaceous beds. 
The great clay-formations, on the other hand, while varying in 
thickness, maintain fairly uniform lithological characters across 
England ; and even in the far north of Scotland, some of the 
Oxfordian clays and Kimeridgian shales are identical in character 
with equivalent beds in the south of England. 
" Tripartite" Series. 
In subdividing the strata, Conybeare and Phillips remarked on 
the apparent regularity of the sequence of clay, sand, and lime- 
stone that characterizes the larger divisions ; and the subject has 
been more fully discussed by John Phillips and others. 
Phillips explains the matter "on the simple and sure basis of inter- 
rupted depression of the sea-bed." He says, " In the cases before us the 
liassic sea-bed first receives only the finest sediments "which can fall in 
deep water ; by degrees these sediments accumulate so as to bring the 
sea-bed near enough to the surface for the drift and settlement of the fine 
sand of Midford and Frocester : on this sandbank flourish colonies of 
coral and shells, and constitute the basis of the Inferior oolite. Depres- 
sion follows ; the deposit again becomes argillaceous ' fuller's earth ' ; 
shallow water succeeds, and the Stonesfield banks of sand and shells 
appear, followed by the Great oolite rock. Less distinctly the same things 
occur and recur ; and the cornbrash ends this^eries." 
" Next we have a long depression markea by 600 feet of Oxford clay, 
followed by the fine sandbank of calcareous grit, on which corals and 
oysters and many forms of life grew in profusion." 
" Again the same things are repeated for the Kimeridge clay, Portland 
sands, and Portland oolite." 
" It deserves remark that the three orders of deposits, clays, sands, 
limestones, are so much alike in the several groups as to be in fact hardly 
distinguishable by hand specimens ; they seem all to have been derived 
from similar sources from neighbouring shores and lands, with no impor- 
tations from afar." * 
* Geology of Oxford, &c., pp. 393, 394. See also Seeley, Physical Geology and 
Palaeontology, 1 885, p. 54. 
