58 
Lower chalk and chalk marl contain more or less alumina and silica, 
a great variation occurring in different localities*. 
Mr. Lyell observes, that ‘‘the area over which the white chalk 
preserves a nearly homogeneous aspect is so great, that geologists 
have often despaired of finding any analogous deposits of recent date ; 
for chalk is met with in the north-west and south-east direction from 
the north of Ireland to the Crimea, a distance of about 1140 geogra- 
phical miles, and in an opposite direction it extends from the south of 
Sweden to the south of Bordeaux, a distance of about 840 geogra- 
phical miles. But we must not conclude that it was ever spread out 
uniformly over the whole of this space, but merely that there were 
patches of it, of various sizes, throughout this area.” (Lyell’s Ele- 
ments, page 401.) 
The Chalk formation of England may be briefly stated as follows, 
and can be well traced on Mr. Greenough’s magnificent Geological 
Map of England and Wales. The strata may be said to commence at 
Dover on the east, at which point the Kentish Downs arise and are 
continued westward until they unite with the North Downs of Surrey. 
On the south-east the chalk again appears in the cliffs of Beachy Head, 
forming there the South Downs of Sussex. The North and South 
Downs of Surrey and Sussex f are continued westward until they join 
* Mr. Bakewell observes in his ‘ Geology,’ page 341, “ Chalk is not, however, absolutely pure, 
for besides the nodules and veins of flint that occur in it, but which bear no sensible proportion 
to the whole mass, some of the strata contain an intermixture with siliceous sand, and in other 
strata calcareous earth is combined with magnesia. In some chalk strata in France, the mag- 
nesia exceeds ten per cent., and I believe many of the English chalk strata contain as great a 
proportion of magnesian earth. 
“Chalk which contains a notable portion of magnesia may generally be known by an appear- 
ance of dendritical spotted delineations on the surface of the natural partings, and by minute 
black spots, like grains of gunpowder, in the substance of the chalk.” 
The appearance of black spots, from my own observation, is more confined to the upper chalk 
than the lower, and well seen at Burpham and Beeding, Sussex. 
+ The South Downs, properly speaking, are only those hills which lie between Hastbourne 
and Shoreham, being twenty-six miles long and seven wide. 
