734 GEOLOGY. 



the most remarkable feature of the deposit. They exhibit every variety of shape, and 

 differ in magnitude from the circumference of an inch to that of a yard. They are com- 

 monly separated from each other, being entirely enclosed by the chalk, and occur in 

 layers horizontally disposed in the rock ; but in one instance a continuous stratum of 

 flint rises from the beach near St. Margaret's Bay, about an inch and a half thick, and 

 may be readily traced from thence for a distance of two miles. But this is an exception 

 to a rule which very generally obtains. The flints in chalk are almost universally 

 detached nodules, with rugged edges, arranged in horizontal strata. These strata are 

 separated from each other by varying distances, and differ also in their own breadth. 

 Upon examination, the flints are frequently found to be siliceous accumulations around 

 organic substances, parts of shells, sponges, and other marine forms, into the most minute 

 pores of which the siliceous matter has penetrated. In many cases the organic structure 

 has been admirably preserved ; but where it has perished, its place has either been left 

 vacant, or filled up with a sparry incrustation. Mr. Brande makes a remark, with a 

 view to throw light upon the enigmatical appearance of the flints, that if finely powdered 

 silica be mixed with other earthy bodies, and the whole diffused through water, the 

 grains of silica have, under certain circumstances, a tendency to aggregate into small 

 nodules. Now grains of quartz are discoverable in chalk, and some conceive it to be a 

 probable explanation of the case, that silex, which occurs in a state of solution in the hot 

 springs of Iceland, was largely present in the waters which deposited the upper chalk, 

 and was aggregated by the elective affinity of its own particles, forming, under control of 

 some peculiar attraction, upon the now enclosed organic substances as so many central 

 nuclei. We may be tempted to inquire, respecting the prevalence of silica in the seas of 

 the period ; but such inquiries are endless, and would be equally pertinent and unanswer- 

 able, concerning the material of the chalk and the magnesia so largely prevalent during 

 the deposition of the magnesian limestone. 



Large masses of limestone are evidently owing to the organic efforts of secreting animals, 

 being entirely composed of comminuted shells $nd corallines ; and a similar organic 

 origin is now generally ascribed to the chalk, for, upon being submitted to microscopical 

 examination, its particles exhibit those appearances of structure which identify them 

 either as excessively minute shells and corals, or portions of them. But if we admit 

 the vital origin of chalk, and of all the enormous masses of carbonate of lime which 

 compose nearly one-eighth part of the superficial crust of the globe, this is obviously no 

 solution of the problem, for we have every reason to suppose that the inhabitants of the 

 ocean secreted the material of their structures from the waters of the deep. The 

 question, therefore, remains to be answered, whence the sea obtained its immense amount 

 of calcareous matter, which appears to have more remarkably characterised it at certain 

 eras than at others. The chief difficulty in the case is, as Dr. Buckland states, that it 

 could not have resulted, like sand and clay, from the mechanical detritus of rocks of 

 the granitic series, because the quantity of lime these rocks contain bears no proportion 

 to its large amount among the derivative strata. Perhaps the true theory may be, that, 

 as lime appears to some extent in lava, basalt, and various kinds of trap-rocks, the ocean 

 obtained its carbonate of lime from springs charged with carbonic acid gas percolating 

 through those igneous masses, and received, at some periods, a preponderating supply 

 from the prevalence of igneous discharges. 



Chalk, with an abundance of flints, presents itself in the cliffs of this formation in 

 the north, and there, on the Yorkshire coast, exposed to the full violence of the 

 German Ocean, the rocks bear witness to the excavating power of the waves, the 

 abrading influence of the stormy wind and rain, and their own comparative fragility. 

 The general contour of the chalk developed in this district has been noticed, that of a 



