fore assumed a position more highly inclined to the horizon than those of a subsequent formation."

83. "Are earthquakes just the shifting of strata while accommodating themselves to a globe which is constantly diminishing in magnitude?"

84. "Was the substance of a vein originally diffused throughout the strata which include the vein, and which has been expressed from the strata after the formation of the fissure which now contains it?"

ON THE FORMATION OF LIMESTONE

BEDS.

85. What is the origin of those beds of limestone which are found associated with the members of the various geological formations? As beds of limestone are not the result of a mechanical deposit, but of a chemical precipitate, must not the formation of those beds be accounted for, not upon mechanical, but chemical principles?

86. It is a fact well known to chemists, that two equivalents of carbonic acid gas, when subjected to a given pressure, will combine with one equivalent of lime, when held in solution, and form the bi-carbonate of lime-a salt that is soluble in water; and should the pressure be afterwards removed, one of

the equivalents of the bi-carbonate will be given off, and the other equivalent will remain in combination with the lime, which is the carbonate of lime—a salt that is insoluble in water-and is, therefore, precipitated. If, therefore, the ocean, at a remote period, held lime largely in solution, as some geologists suppose, and carbonic acid gas escaped then, as it does now, from clefts or fissures in the crust of the earth, it is evident, that if, at a great depth, this gas escaped from a fissure at the bottom of the ocean, it would combine with the lime in solution, and, under the great pressure of the superincumbent waters, would form the bi-carbonate of lime-a salt that is soluble in water. Now, as every soluble body has a tendency to diffuse itself through the menstruum in which it is dissolved, the bi-carbonate of lime, thus formed, would diffuse itself in the waters laterally and vertically, the vertical diffusion being co-extensive with the lateral diffusion; but during the diffusion of the bi-carbonate of lime towards the surface of the ocean, the superincumbent pressure of the waters would be gradually removed, until, at length, the bi-carbonate would part with one of its equivalents of carbonic acid gas, and would thus be reduced to a carbonate of lime, which, not being soluble in water, would be preci

pitated and hence a bed of limestone, co-extensive with the lateral diffusion which had taken place in the ocean of the bi-carbonate of lime, would be formed.

87. During the process, which we have just described, by which the bi-carbonate of lime is reduced to a carbonate, a quantity of carbonic acid gas is given off, precisely equal to the quantity of carbonic acid gas which is precipitated with the lime in the formation of a limestone bed; that while one equivalent of carbonic acid gas is precipitated with the lime, the other equivalent of the gas is given off, to enter into the composition of the atmosphere.*

88. As the carbonate of lime is insoluble in water, whence was it that the encrinites of the mountain limestone derived their carbonate of lime? Was it from a bi-carbonate of lime which the ocean held in solution that those encrinites derived their calcareous matter? Was the bi-carbonate decomposed, of which one equivalent of carbonic acid gas in combination with the lime was appropriated by

* An analogous process takes place in calcareous springs which are charged with the bi-carbonate of lime-one equivalent of carbonic gas is given off to the atmosphere, and the other equivalent in combination with the lime is precipitated.

the encrinites, while the other equivalent of carbonic acid gas was given off to the atmosphere?

89. If so, was it during the formation of the enormous beds of mountain limestone, when the bicarbonate of lime, according to the process we have described, was reduced to a carbonate, that the surplus carbonic acid gas, which would thus be given off to the atmosphere of the carboniferous period, would be fully adequate to provide for the growth of the luxuriant vegetation of that period?

90. How is it that the limestone which occurs in primitive rocks seldom consists of extended beds, but is found in lumpish masses that are included in those rocks? Is it from this cause-the primitive limestone was deposited in shallow water, when the ocean enveloped the whole globe; and because of this, when carbonic acid gas escaped from a fissure at the bottom of the ocean, the pressure of the superincumbent waters was not such as to cause the carbonic acid gas to combine with the lime in solution to form a bi-carbonate, but a carbonate of lime? There would be thus little or no lateral diffusion, as the carbonate of lime, when formed, would be immediately precipitated, and thus lumpish masses of limestone would be formed, and not extended beds.

ON THE FORMATION OF MURAL CLIFFS.

91. There is one characteristic of mural cliffs, which, so far as my observation goes, is common to all, whether those cliffs occur inland or upon the sea-shore. At the base of those cliffs, and along the whole extent of the base, a stratum of rock occurs of softer consistency than the superincumbent rock. When, therefore, a series of strata, somewhat inclined to the horizon, is situated upon the seashore, with their escarpments exposed to the action of the waves, and the inferior stratum is composed of softer material than the superincumbent rocks, the stratum of softer consistency undergoes a more rapid abrasion than the rocks above; and because of this, those rocks are left unsupported, and from time to time give way, and thus a mural cliff, with a perpendicular face, is formed.

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92. On those parts of the coast where the sea has made rapid encroachments upon the dry land, I have observed that this occurs more particularly where a cliff rests upon a thin stratum of rock, of which the consistence is of softer material than the rock of which the cliff is composed. The sea ex