12 LOWER OOLITIC ROCKS OF ENGLAND : 
these appear to be rolled fragments of the calcareous mud which 
was accumulating under the influences of the decomposition and 
attrition of calcareous organisms, combined it may be with Gir- 
vanella-growth and the precipitation of minute calcareous particles 
from solution. 
There may be recognized then in the formation of oolitic grains 
the operation of a variety of causes : 
(a.) Mechanical movements. 
(6.) Organic growth, e.g., Girvanella. 
(c.) Deposition of carbonate of lime from solution. 
The interstices may have been filled up at the time by cal- 
careous mud, or subsequently by the deposition of calcite. In a 
few rare cases the rock is made up of the larger fragments of oolitic 
grains, and rolled organic fragments without any cement at all. 
Thus, in an example of oolite (Lincolnshire Limestone) from Weldon, 
there was no matrix. The oolite grains were joined together at 
the points of contact, and their external surfaces were rough in 
consequence of the projection of excessively minute crystals of 
calcite.* 
As regards those oolites which contain a large amount of 
carbonate of iron of the type of the Cleveland ironstone, and the 
blue varieties of Northamptonshire ironstone, there seems no reason 
to doubt that they have originated, as Dr. Sorby has maintained, 
by the subsequent replacement of carbonate of lime by carbonate 
of iron. 
The calcareous rocks with detached oolitic grains, composed 
mainly of ferric oxide, are difficult to account for on the hypothesis 
of replacement subsequent to formation. J. J. H. T. 
Sandstones. 
The sandstones, of which examples are found in the Inferior 
Oolite, Stonesfield Series, Forest Marble, Corallian Beds, 
Kellaways Beds, Portland and Purbeck Tieds, consist of angular 
and sub-angular grains of quartz, cemented by a matrix of 
crystalline or granular calcite. 
In a fine-grained sand (Midford Sand) from Seizincote near 
Stow-on-the-Wold, J. A. Phillips noted the occurrence of grains 
of sand, which are generally angular, but in some instances " their 
more acute angles appear to be slightly rounded." He observed 
also numerous fragmentary crystals of schorl and garnet, t The 
ferruginous sandstone of Duston, which was examined by Mr. 
Teall, showed angular quartz grains, occasionally also flakes of 
white mica, and probably some felspar, cemented by ferric oxide, 
and with some carbonates in places. 
Concretionary masses of Sandstone or Doggers occur in the Mid- 
ford and Northampton Sands, in the Collyweston and Stonesfield 
Slate series, in the Forest Marble (Hinton Sand), in the 
* See also A. Harker, Naturalist, 1890, p. 302. 
t Quart. Journ. Geol. Soc., vol. xxxvii. p. 16. 
