JUKES-BROWNE : MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 393 
general similarity of the deposit to that now forming in certain parts 
of the Gulf of Mexico and the Florida Straits, and especially in 
depths of from 200 to 500 fathoms, according to Dr. J. Murray.* 
Again, the assemblage of arenaceous Foraminifera is best matched in 
depths of from 350 to S00 fathoms at the present time.f If this 
conclusion is correct we may safely assume that the rest of the Lower 
Chalk was formed in rather deeper water, probably from 400 to 600 
fathoms. 
The Melbourn Rock and zone of Rhynchonella Cuvieri do not 
afford much positive evidence. The absence of glauconite suggests 
distance from land, but the presence of large quartz grains is puz- 
zling. The stratigraphical and palaBontological evidence connected, 
the Belemnite Marls and Melbourn Rock indicate the occurrence of 
considerable geographical changes at this time, affecting the direction 
and strength of the marine currents and largely altering the fauna. 
All the facts harmonize with the inference that these changes were 
brought about by continued subsidence. 
Conditions then seem to have settled for a time, and the struc- 
ture of the central part of the Middle Chalk seems to indicate the 
quiet deposition of purely calcareous ooze in deep water at some 
distance from land. The almost entire absence of mineral particles 
in the Ter. gracilis zone is a striking fact, for there are few modern 
oozes which are so free from minerals. The absence of pumice and 
the extreme rarity of augite and olivine is proof that no active 
volcanos existed within many hundred miles of our area, and it is the 
absence of this volcanic material which makes the Chalk seem so 
much purer than modern Globigerina ooze. The mass of the Chalk 
of the Ter. gracilis zone seems to be entirely made up of fine powder 
derived from the shells of Foraminifera, Calcareous spheres, Deep- 
sea Echinoderms, certain Mollusca and Brachiopoda, and Coccoliths. 
The Chalk Rock presents a great contrast to this ; the recurrence 
of glauconite and of quartz in Southern England suggests a partial 
re-elevation of land, and there are other reasons for thinking that 
there was an upheaval of the western island at this time. The character 
* Bull. Mus. Comp. Geol. , Harward Coll., U.S., vol. xii., p. 52. 
t W. F. Hume, Op. Cit., p. 81. 
