ON THE CRYSTALS IN LAVAS. 



177 



an examination of this question, deduced from the true 

 state of things, aad carried by facts to a degree of evidence 

 that excludes all doubt, would decide one of the most im- 

 portant points in geology, by exhibiting a just idea of vol- 

 canoes and their phenomena. 



The principal avgument of Mr. Fleuriau de Bellpvue is Crystals of glass 

 drawn from the analogy he finds between the formation of ^^^^i^f founda- 

 the crystals contained in lava, and that kind of crystalliza- tem. ^ 



tion, which has been called crt/sfallites, and is formed in the 

 pots in glass-houses, when the glass that was in fusion is suf- 

 fered to cool slowly. 



Let us now examine what these crystallites of glass- These crystals 

 houses are. The whole mass of cooled glass exhibits aeon- ^^^" ^ * 

 fused crystallization, all of the same tint, in which we see 

 small compact bars confusedly interlaced^ some slightly 

 striated, aad others disposed in the form of stars, equally 

 confused. At other times a number of threads are formed 

 at the bottom of the pot, which cross and intermingle with 

 each other, and exhibit likewise stellate figures. 



In the first case these crystallites compose the substance 

 of the glass, and are distinguished only in some places. In 

 the second we see at the bottom, through the transparent 

 glass, these bundles of nets, and starry figures, which have 

 some resemblance in shape to the little icy stars, that fail 

 with snow in very cold weather. Perhaps some instances of 

 more decided vitreous crystallizations may occur : but these, 

 which are rare, only prove, that there may be some circum- 

 stance favouring this crystallization in a very small space. 



Mr. Fleurieu de Bellevue conceives, that these crystal- These said to 



lite figures sittgiilarlyres€mbietYem.o\ite. This opinion, that ^^^^I^L .^ '^^° 



there exists a striking resemblance between two substances, 



one of which is the product of a glass-house, the other of 



a mineral stratum, appears to me astonishing I confess; for but they have 



in this way there is no substance, which we may not i-eckon i^erely some 

 . ., ' , -11,1 •'..,.. similaiity la 



Similar to another, provided they have some similarity in shape. 



form. Thus we may say, that the capillary schoerls, or 



mineral byssus, i-esemble hairs ; and that the fibres of the 



amianthus, or stone-flax, resemble those of flax or hemp; 



though these substances merely resemble each other in 



form, without there being any real similarity between them. 



Vol. XX.— July, 1808. N This 



