386 JUKES-BROWNE : MICROSCOPIC STRUCTURE OE ZONES OF CHALK. 
(3) It must also be remembered that the original structure of 
some parts of the Chalk may have been different from that of any 
deposit which could be formed at the present time. Chalk is mainly 
of organic origin, and the organisms of the Cretaceous period were 
different from those which now exist, and, as some of the Chalk- 
builders belong to extinct genera, we cannot be sure that their habits 
and habitats were similar to those of their nearest living allies. This 
is especially the case witli the Mollusca. The Foraminifera, and 
especially the arenaceous forms, are more likely to have been similar 
in these respects to their modern congeners. 
(4) Some of those who have discussed the origin of the Chalk, 
as a whole, have been more or less biassed by preconceived views 
regarding the permanence of continents and ocean basins, and have 
therefore sought to minimise the resemblances which some parts of 
the Chalk present to modern deep-sea deposits ; while their opponents 
have naturally made the most of these resemblances. The problem 
of the Chalk should be considered by itself, and the evidence must 
be balanced by a mind which is free from the bias of any previous 
theoretical views regarding the origin and growth of oceans and 
continents. 
In the present communication I propose to follow one line of 
investigation only, and to point out the inferences which may be 
deduced from the facts so far as they are at present known to me. 
This branch of the subject is the minute structure of the successive 
zones of the Chalk as revealed by the microscope. Most of my in- 
formation on this subject is derived from the work of my friend, 
Mr. W. Hill, published in several papers and in the Journal of the 
Geological Society of London, but an important contribution has 
recently been made by Dr. W. F, Hume, in his essay, entitled 
" Chemical and Micro-minerological Researches on the Upper Cre- 
taceous Zones of the South of England," 1893. 
2. Constituents of Chalk. 
Before entering into particular descriptions it will be desirable to 
give some account of the general structure of Chalk and of its prin- 
cipal constituents. Most kinds of Chalk have a matrix of more or 
