10 LOWER OOLITIC ROCKS OF ENGLAND : 
Girvanella type, has played an important part in the formation of 
the concentric and radiate portions of oolitic grains ; but the 
growth of minute Algse, as suggested by Dr. Rothpletz, may have 
influenced the formation of the grains. 
Oolitic grains vary considerably in the character of their nuclei. 
These may consist of quartz grains, amorphous pellt ts, pellets 
with Girvanella-slructure, fragments of oolitic grain?, organic 
fragments (Foraminifera, and fragments of Mollusca, Corals, and 
Echinoderms), complete oolitic grains with amorphous material, 
&c. 
The structure of the pisolitic grains differs from, that of the 
typical oolitic grains, for they exhibit a rude kind of concentric 
structure, and radial structure is absent. It is in these 
grains that the Girvanella-structure is best seen. Assuming 
Girvanella to have been an incrusting organism, the feature of 
these grains may be explained by supposing the interstices 
between the Girvanellse-tubes to have become filled up with cal- 
careous mud. That the pisolites have picked up foreign matter is 
proved by the occasional occurrence of sand-grain? in them. 
So far reference has been made only to calcareous oolitic grains. 
In some rocks the oolitic grains are formed of concentric layers 
of deep brown ferric oxide. In these oolites the nuclei often 
consist of broken oolitic grains. In other ferruginous oolites 
(e.g., Cleveland) rhombs of carbonate of iron are present and the 
true structure of the interior portions of the grain has been lost. 
An example of silicified oolite has also been obtained by Mr. 
Woodward from the cherty Portland Beds of Dorsetshire. 
II. Organic Fragments. 
The Organic remains that have been found in the Oolitic rocks 
include Calcareous Algse, Foraminifera, Ostracods, and fragments 
of Corals, Echinoderms, Annelides, Polyzoa, Brachiopods, and 
Mollusca.* Such remains have been observed by Dr. Sorby and 
Mr. E. T. Newton. In some cases the original structure of the 
organic remains is preserved ; in others it has been destroyed by 
re-crystallization. 
III. Non- calcareous Detrital Material. 
Among the residues of the Oolitic rocks, in addition to Quartz, 
I have found Oligoclase, Plagicclase, Rutile, Tourmaline, 
Garnet, and Zircon. The insoluble residues of the Inferior Oolite 
of the Cqtteswold Hills have been studied by Mr. E. Wethered ; 
and he shows that the beds, from the Pea Grit Series to the Rag- 
stone?, contain from 1 to 5 per cent. He notes mica, and silicate 
of alumit a, as well as some of the minerals above noted. f 
IV. Matrix. 
Reference has already been made to the general characters of 
the matrix ; one feature, however, deserves special mention. 
* For illustrations, see Carpenter, Eep. Brit. Assoc. for 1844, p. I ; 1848, p. 93. 
f Quart. Journ. Geol. Soc., vol. xlvii. p. 559. 
