A. J. Jukes-Browne—Silica in Chalk. 543 
Are we to suppose that we have here a case of the arrested 
development of flint? Possibly, but it would be very rash to assume 
that the Dorset Lower Chalk was originally a siliceous Chalk 
similar to that of Wiltshire. Nor is the latter the most siliceous 
Chalk known, for some beds at Collingbourn Kingston, near Andover, 
in Hants, contain over 88 per cent. of such silica. The following 
analysis of Collingbourn Chalk I owe to the kindness of Professor 
J. B. Harrison, of Demerara, who adopts a special method for 
separating the free colloid silica from that existing in combination 
with bases such as Alumina and Iron.’ The analysis is consequently 
of special value and accuracy :-— 
Anatysis or Srnrckous CHALK, CoLLINGBOURN. 
Organic matter, etc. ane boc boc seoral RO O¥s 
Quartz... 50¢ 200 ae co ween ose 
Colloid silica ooe & ae oo we 39°69 
Silicates (Clay, etc.) S00 000 300 we. «14°75 
Calcium sulphate ... one oor coc ooo, Pa) 
Calcium carbonate 500 soe oe cos, OPT 
Calcium phosphate 208 cot p00 .-. trace 
99°80 
Here then is a Chalk which actually contains more colloid silica 
than carbonate of lime; if flints were formed by the subsequent 
solution and concentration of such silica, how is it that the Colling- 
bourn Chalk is not now a pure Chalk with frequent bands of flints ? 
Evidently the formation of flints is not a matter of simple subsequent 
concentration, neither does it depend simply upon the abundance of 
disseminated colloid silica. 
2. The Middle Chalk is that which lies between the horizons 
known as the Melbourn Rock and the Chalk Rock. It is, so far as 
we know, a very pure Chalk. The few analyses that have been 
made show that it contains from 97 to 98 per cent. of carbonates, 
and that it is therefore as purely calcareous as many parts of the 
Upper Chalk. 
The distribution of flints in the Middle Chalk is somewhat 
remarkable: they appear to be absent in the Isle of Wight and in 
North Dorset; they occur sparingly in the central zone in Wiltshire, 
and increase in abundance northward till in North Cambs and Suffolk 
they form frequent layers of nodules. The character of the Chalk 
composing the zone of Terebratulina gracilis varies, however, very 
little; it consists largely of single calcareous cells with a few 
Foraminifera embedded in a matrix of very finely-divided calcareous 
ooze. Sponge spicules are rare, and no globular silica has been 
observed. This description applies equally to the flintless Chalk of 
the south and west, and to the portion containing flints in the 
counties of Hertford and Cambridge. The following is an analysis 
of Middle Chalk with flints from 60 feet above the Melbourn Rock 
at Hitchin; it has been kindly made for me by Prof. Harrison with 
the express purpose of ascertaining the amount of colloid silica. 
1 Quart. Journ. Geol. Soc. vol. xly. p. 414. 
