394 JUKES-BROWNE : MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 
of the Chalk Rock fauna confirms the above inference, for it abounds 
in Gasteropoda and recalls that of the Chalk Marl. If the depth of 
the water over Hampshire had increased to 700 or 800 fathoms 
during the epoch of the Ter. gracilis zone an uprise of the south- 
western land may have brought its coast line within 100 miles with- 
out decreasing the depth of water by more than 100 or 150 fathoms, 
and without bringing the eastern shore of the Cretaceous Ocean any 
nearer our area because the upheaval may have been a tilting move- 
ment only lifting the western islands. 
From this time we seem to be following the progress of a great 
and extensive subsidence. The grains of glauconite become smaller 
and fewer and eventually die out altogether ; the same may be said 
of the detrital minerals, only a few minute angular particles of quartz 
being found in the higher beds of the Upper Chalk, quite invisible in 
a slice even under a half-inch lens. Arenaceous Foraminifera are 
also absent except Ammodiscus incertus. 
The mere thickness of deposit above the Chalk Rock amounts to 
200 fathoms in the Isle of Wight, and the fineness of the material 
shows that its accumulation was very slow. Its other characters 
indicate distance from Continental land and a considerable depth of 
water. At the same time the deposit is purely calcareous and shows 
no tendency to pass into a Red Clay ; this however is due to the 
absence of volcanic material and is not a reliable indication of depth, 
for Globigerina oozes, with over 90 per cent, of Calcium Carbonate, 
occur in the South Atlantic at depths of about 2000 fathoms. It is, 
therefore, quite possible for a purely calcareous deposit to be formed 
at that depth. 
It has been asserted that Coral-mud is more like Chalk than any 
other modern deposit, but the resemblance is probably quite super- 
ficial. Dr. J. Murray describes Coral-mud as always containing the 
debris of reef-forming organisms, such as Corals, Calcareous Algae, 
Echinoderms, Alcyonaria, Pteropoda, small Gasteropoda, and other 
Mollusca. I need not say that such an assemblage is not to be found 
in any part of the Chalk. The only bed which presents any approach 
to such a structure is the Chalk Rock, but few corals and no Alcyo- 
narians or calcareous Algae have yet been observed in that. Prof. 
