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POPULAR SCIENCE REVIEW. 
During the latter part of that portion of time known on 
the Continent as the Upper Jurassic Period, in which our so- 
called Middle and Upper Oolites were formed, the greater 
part of what is now France and England was submerged 
beneath an open sea, into whose depths fell gradually the fine 
argillaceous deposits of the Oxford and Kimmeridge Clays. 
These were doubtless derived in large measure from the denu- 
dation of the Lias, which spread in former times far to the 
west of its present boundary, and whose worn edges were 
covered in Dorsetshire and Devonshire by the deposits of the 
next subsequent submergence, the Greensand and Chalk. This 
open Jurassic sea did not retain an unbroken tranquillity ; 
again and again was its floor upheaved, or other disturbances 
took place which left their marks in the changed nature of 
the rocks, and these changes were not continuous over the 
whole area, but were limited now to one locality and now to 
another. Nor did they take place approximately at the same 
time ; for while the clay was still being deposited in one place, 
we learn both from actually tracing the rocks and by exa- 
mining their fossil contents that limestone or sand were being 
formed at another. To these local changes and their resulting 
deposits the name of episodes has been assigned ; and thus 
we may speak of the Portland rocks as representing the later 
of the two episodes which in our own country affected the 
upper Jurassic seas. It has been the custom amongst Con- 
tinental geologists to call by the same name the later of two 
episodes that have affected their particular country; that is, 
the upper mass of limestone found in the midst of their clays 
has been called Portland. These, however, have nothing in 
common with our own except the order in which they have 
occurred. They are all of greater antiquity ; and nowhere out 
of England are true Portland limestones found except at 
Boulogne and, as is reported, in the Pays de Bray. 
In the general rise of the earth’s surface which closed for 
us the Jurassic period, the area surrounding the English 
Channel appears to have remained longest below the sea, and 
to have contained the last deposits of that era. There is some- 
thing peculiar, therefore, about the circumstances of the deposi- 
tion of our Portland limestone which is of interest in itself. 
Moreover, of all the beds which pass by that name in England 
amongst geologists, that which has been celebrated amongst 
builders is the youngest. 
This will be seen by a brief account of the ‘ Portland Stone ’ 
as a building stone, as it is worked in the various parts of the 
country where it occurs, beginning at the north, and ending at 
the typical locality in the extreme south. There is no great 
thickness of these rocks at their most northerly localities, 
