Dr. FiTTON on the Strata below the Chalk. 121 



variation in the circumstances under which the different strata were deposited, 

 although some of those traversed by the lines above described alternate with 

 others, in which no traces of them, or scarcely any, are to be seen. 



(23.) The concretional masses described in the preceding sections give 

 rise to some questions of great interest in geology. There can be no doubt 

 that all these strata were originally deposited in the state of sand and gravel ; 

 but in the quarries of East Kent all stages of gradation can be perceived, 

 from distinctly separated concretions of stone*, to others so nearly uninter- 

 rupted, that the next step into perfect continuity can be easily conceived. 

 The sands below the Portland-stone the coral-rag and the inferior oolite, 

 afford examples of the same character : and in all these cases the concretions 

 must have been formed after the deposition of the sand which includes them, 

 and probably beneath a great depth of compacted materials. In such a mass, 

 shut off from the free access of air and change of temperature, there is no 

 obvious reason for disturbance of the affinities which maintained the original 

 form of the components, except the decomposition of the animal and vege- 

 table remains diffused among the stony substances ; yet here we have not 

 only solid limestone, where nothing but loose sand and gravel were before, 

 but firm siliceous concretions, pervading, and so identified with, the separated 

 particles of the quartzose sand, that the whole is nearly homogeneous. If the 

 decomposition of the organized substances, continually acting throughout very 

 long periods of time, be not sufficient to produce the whole of these effects, 

 perhaps it may be supposed that electricity is the cause which sets free the 

 elements, and disposes them to combine anewf. 



(24.) The second, or middle subdivision (6.) of the lower green-sand, first 



* Concretions nearly globular occur, in the uppermost yellowish sands, at Parkhouse, near 

 Folkstone. 



t In preparing nitric acid in a chemical laboratory where I was attending some years ago, the 

 glass retort broke towards the close of the operation, and a quantity of the concentrated solution 

 of bisulphate of potash oozed out through a very fine crack into the surrounding sand. On re- 

 moving the retort some time afterwards, a large part of the sand was found to be agglomerated into 

 beautiful concretional masses, like bunches of grapes, the greater portion of which, of course, was 

 sand, and the cementing substance the crystallized salt. The masses of gravel frequently found 

 cemented by carbonate of lime, or of iron, in consequence of the escape of the carbonic acid which 

 had held those substances in solution, is an example of another mode of concretion. In the first 

 of these two cases the loss of heat, in the latter the escape of a chemical solvent, has obviously 

 been the cause of the consolidation. But the case mentioned in the text is distinct from both. 



Dr. Turner justly remarks, respecting substances usually considered as insoluble, that " although 

 " the weight of such bodies is not perceptibly diminished by trials conducted in the laboratory, 

 " during a short interval of time, and with small quantities of water, the effect of the same opera- 

 VOL. IV. SECOND SERIES. R 



