385 
THE MICROSCOPIC STRUCTURE OF THE ZONES OF THE CHALK. 
BY A. J. JUKES-BROWNE, B.A., F.G.S. 
Read April nth, 1891 
Introductory. 
It is somewhat curious that such diverse views should be held 
by different geologists respecting the origin of the formation known 
as the Chalk, and regarding the conditions under which it was accu- 
mulated. This difference of opinion is probably owing to several 
distinct causes, and I think that before anyone can hope to arrive at 
a j nst and sound conclusion on this much debated question, he should 
fully and fairly consider the reasons why previous investigators have 
been led to take such different views. There are at least four causes 
which have conduced to this divergence of opinion. 
(1) The minute structure of the Chalk has not been sufficiently 
investigated The Chalk is not a simple formation of moderate 
thickness and extent like many other limestones, nor is it of similar 
composition throughout. Its thickness is great, 1,700 feet in the 
Isle of Wight, and its component strata are found to differ much 
from one another when they are examined with the microscope, 
though to the eye they may seem to have a very similar structure. 
Consequently, the inspection of a few slides of Chalk will not reveal 
the history of its formation, and no one ought to discuss the origin 
of the Chalk, as a whole, unless he has examined a large series 
of slides from different horizons and from different areas of Chalk. 
(2) "When any kind of Chalk is compared with modern deposits, 
either chemically or microscopically, it should be remembered that 
certain chemical and minerological changes must have taken place 
during the consolidation and upheaval of the formation. Some of 
these changes probably occurred while the formation was still below 
the Cretaceous Sea, and others were accomplished by the percolation 
of subaerial water through the mass. Some ingredients, such as the 
organic silica, the oxides of iron and manganese and the alkalis, have 
been dissolved and re-distributed or concentrated into concretions. 
