Not only are mineral philosophers so inconsiderate, in forming geological theories upon a mere supposition or false analogy, they have even proceeded, upon that erroneous theory, to form a geological supposition for explaining the appearances of strata and other stony masses in employing a particular physical operation, which is, that of _crystallization_[37]. Now crystallization may be considered as a species of elective concretion, by which every particular substance, in passing from a fluid to a solid state, may assume a certain peculiar external shape and internal arrangement of its parts, by which it is often distinguished. But, to suppose the solid mineral structure of the earth explained, like an enigma, by the word _crystallization_, is to misunderstand the science by which we would explain the subject of research; and, to form a general mineral theory thus upon that term, is an attempt to generalise without a reason. For, when it were even admitted that every solid body is crystallised, we thus know no more of the geology of this earth, or understand as little of the general theory of mineral concretion, as we did before;--we cannot, from that, say whether it be by the operation of solution or of fusion which had produced the perceived effect. [Note 37: Journal de Physique; Avril 1753.] M. de Carosi has wrote a treatise upon certain petrifactions[38]. In the doctrine of this treatise there is something new or extraordinary. It will therefore be proper to make some observations on it. [Note 38: Sur la Génération du Silex et du Quartz en partie. Observations faites en Pologne 1783, à Cracovie.] The object of this treatise is to describe the generation of silex and quartz, with their modifications or compositions, formed within mineral bodies of a different substance. The natural history contained in this little treatise is well described and sufficiently interesting. But It is chiefly in order to examine the means which, according to the theory of this treatise, are employed in petrifying bodies, that I consider it in this place. The first section of this treatise has for title, _Génération du Caillou et du Quartz de la terre calcaire pure_. It may be worth while to compare the natural history of this part of the earth with the flint and chert found in our chalk and lime-stone countries. I shall therefore transcribe what is worth observing upon that subject (p. 5.).