300 



THE WONDERS OF GEOLOGY. Lect. IV. 



calcareous cases and shells of the animalcules termed Fora- 

 minifera* which swarm in inconceivable numbers in our 

 present seas, and are daily and hourly contributing to the 

 amount of sediment now accumulating in the bed of the 

 ocean. 



5. Flint nodules and veins. — The flints (as the sili- 

 ceous nodules are generally termed) occur in horizontal 

 rows or layers, generally parallel with the lines of stratifi- 

 cation of the chalk, but placed at unequal distances from 

 each other. They are of all sizes, from that of a small nut 

 to masses several feet in circumference. But besides these 

 parallel beds, there are sheets of flint intercalated between 

 the chalk strata, and covering areas, often many miles in 

 extent. These continuous layers are termed tabular flint ; 

 they are seldom more than two or three inches in thickness, 

 and veins and threads of silex frequently ramify from them 

 into the fissures of the surrounding chalk. This tabular flint 

 is generally solid throughout, but sometimes incloses a layer 

 of chalky matter, full of the usual minute organisms of the 

 chalk. Vertical and diagonal veins of flint filling up cre- 

 vices and fissures in the rock, and traversing both the 

 strata of limestone and any sheets of tabular flint that may 

 be interposed, are of common occurrence ; these prove that 

 the lower beds were consolidated, and had subsequently 

 been fissured, before the superimposed strata were deposited, 

 and the streams of siliceous matter flowed over them. 



The nodules and veins of flint that are so abundant in 

 the upper chalk, have probably been produced by the 

 agency of heated waters and vapours. The perfect fluidity 

 of the siliceous matter before its consolidation, is proved 

 not only by the sharp moulds and impressions of shells, &c. 

 retained by the flints, but also by the presence of numerous 

 organic bodies in the substance of the nodular masses, and 

 the silicified condition of the sponges and other zoophytes 

 * Medals of Creation, vol. i. p. 228. 



