1852.] BROWN UPPER TERTIARIES AT COPFORD. 191 



Ha^dng thus described the Copford deposits in detail, I would 

 observe that the sandy gravel. No. 5, appears to me to belong to the 

 Till or Boulder-clay. The blue-clay or brick-earth (No. 4) I have 

 long considered to be a modification of that deposit on account of its 

 organic and mineral contents, which have originally been derived from 

 a distance; and the brown clay with boulders also (No. 1) certainly 

 contains evidence of an origin identical with that of the boulder-clay. 

 The angular flints and sandstone-boulders especially, scattered over 

 the surface here, are similar to those of the Till of the neighbouring 

 parts of Essex. From the Till of Bures, and in the cutting of the 

 Stour Valley Railway, fossil remains have been obtained in great 

 abundance, together with boulders and angular fragments derived 

 from various of the older formations. The Till continues southward 

 from Copford to near the coast, at places assuming a more chalky 

 character. 



List of Rocks and Minerals forming Boulders and Gravel in the 

 Till of Copford and its neighbourhood"^ . 



London Clay Septaria. Copford. 

 Nodules of Pyrites and Crystals of Selenite. 

 Greywether Sandstone (one angular block weighing 30 cwt.). 

 Hertfordshire Puddingstone, in large angular fragments. 

 Chalk, in large boulders. 



Unrolled chalk-flint and large angular flints. Copford. 

 Limestones, with and without fossils (Oolite, Lias, &c.). Copford. 

 Hard Lias Limestone, grooved and polished. 

 Porous siliceous rockf ; abundant in the Till and gravels of the 

 neighbourhood. 



* For a list of Rock-specimens from the TiU of Ballingdon Hill, Essex, see Mag. 

 Nat. Hist. 1836, vol. ix. p. 43. 



t Mr. Sorby has obligingly favoured us with some remarks on this peculiar 

 rock. This rock, he observes, is one of great interest, having such a structure as 

 to well deserve a detailed description, which I shall probably give on some other 

 occasion. At present I need only say that it appears to have been, originally, 

 similar to one of those varieties of limestone which consist to a great extent of 

 fine rounded grains of calcareous organisms, more or less slightly coated with a 

 layer of the same in a more finely comminuted condition, along with a little 

 fine clay, so as to form an incipient oolite, the grains of which have not received 

 so thick a coating as to develope a genuine oolitic structure. I am not acquainted 

 with any of exactly the same character as this appears to have had, but some of 

 the oolites in Lincolnshire are very closely allied to it. Its present condition has 

 resulted from silicification, which has removed the calcareous matter, and left 

 the spaces, originally occupied by the round oolitic grains, either empty or filled 

 with agate. The sides of those which are empty are covered over with radiate 

 crystals of agate ; and the surrounding mass is transformed into a somewhat 

 similar structure. The rudimentary oolitic grains were on an average about ■^^'^^ 

 of an inch in diameter, and about one-half are now empty, which gives the rock 

 so porous a character. It also contains fragments of agatized shell with the 

 structure very perfectly preserved. 



Mr. S. has also examined two other forms of concretionary structure in rock- 

 fragments from bed No. 5 of the Copford section, one of which is silicified, and, 

 in Mr. Sorby's opinion, a pseudomorpb ; and the other presents the form of oolitic 

 structure termed by Mr. S. ** positive concretionary." He considers it probable 

 that both have been originally formed from chalk-debris. 



o2 



