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POPULAR SCIENCE REVIEW. 
tions. The lower part of the ‘ stone/ resting at a considerable 
elevation on the * sand/ is very flinty and of no use for build- 
ing, though it represents, with the single exception above 
noted, the building stones of all the other districts. It is a 
remarkable circumstance that the flint is only developed at 
those places which have limestones of the same series overlying 
the beds, and the more of this overlying limestone there is 
the more flint is found. It is as if the flint were derived from 
this limestone and passed from it into the beds below. The 
upper part, however, of this flinty series contains beds which 
yield a very good stone, though not the best. Here the flints 
are few and far between, and only spoil the six or eight inches 
in which they lie. These beds are very inconstant, and, in- 
deed, are more or less false-bedded; they increase in a few 
hundred yards to a workable thickness, and in a few hundred 
more become almost worthless. 
The most valuable stone is known as the Whit or White 
bed ; towards the east of the island this is seen to become 
false-bedded and to thin out, lying on a worn surface of the 
older series, but towards the west it attains a thickness of eight 
or ten feet, and is at its best. The grains of which this lime- 
stone is ordinarily composed are very small, and probably 
represent minute fragments of shells, though towards the top 
it is partially oolitic. Its qualities are too well known to need 
description, one of its chief peculiarities being its freedom from 
fossils ; blocks of it give a fine bell-like tone, not, however, 
superior to that of the Tisbury stone. Over this in the island 
alone is found a remarkable rock, known locally as the ‘ roach/ 
It is wholly composed in one sense of shells, and yet in another 
sense there is not a shell in it. Originally it was a mass of 
shells, chiefly a doubtfully marine species, Cerithium port - 
landicum , or the ‘ Portland Screw/ These lay as close to one 
another as they could, the intervening spaces only being filled 
with some calcareous debris , subjected subsequently to some 
process similar to that which has dissolved the silex out of so 
many fossil sponge spicules and deposited it around them ; the 
shells have all been dissolved, and only that part of the rock 
remains which was not shell, that is, the surrounding matrix 
and the internal cavity, so the roach is full of the holes where 
the shells have been, and contains models of their outside and 
inside. In this central spot — the last to emerge from out of 
the fostering sea — the enfeebled inhabitants of the period were 
congregated, and here they were together overwhelmed and 
found a common grave. 
Although at so short a distance from Portland, neither 
Up way nor the Isle of Purbeck yield such good stone as the 
island. At the former place there is very little even to repre- 
