Theory of the Earth

subject on which to reason, for ascertaining the order of production in certain bodies. This body, which we are now to examine, is of the agate species.

WE have now been considering the means employed by nature in consolidating strata which were originally of an open structure; but in perfectly solid strata, we find bodies of agate, which have evidently been formed in that place where they now are found. This fact, however, is not still that of which we are now particularly to enquire; for this, of which we are to treat, concerns only a cavity within this agate; now, whatever may have been the origin of the agate itself, we are to shew, from what appears within its cavity, that the crystallizations which are found in this place had arisen from a simply fluid state, and not from that of any manner of solution.

THE agates now in question are those of the coated kind, so frequent in this country, called pebbles. Many of these are filled with a siliceous crystallization, which evidently proceeds from the circumferences towards the centre. Many of them, again, are hollow. Those cavities are variously lined with crystallized substances; and these are the object of the present examination.

BUT before describing what is found within, it is necessary to attend to this particular circumstance, that the cavity is perfectly inclosed with many solid coats, impervious to air or water, but particularly with the external cortical part, which is extremely hard, takes the highest polish, and is of the most perfect solidity, admitting the passage of nothing but light and heat.

WITHIN these cavities, we find, first, The coat of crystals with which this cavity is always lines; and this is general to all substances concreting, in similar circumstances, from a state of fusion; for when thus at liberty they naturally crystallize. 2dly, We have frequently a subsequent crystallization, set upon the first, and more or less immersed in it. 3dly, There is also

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