178 



Tremolite de- 

 scribed. 



Cannot have 

 been crystal- 

 lized from fu- 



Crystals in 



KavAS. 



Several kinds 

 of them, dif- 

 ferent from 

 each other,and 

 from the lava. 



ON THE CRYSTALS IN LAYAS. 



This remark was necessary, as it might be supposed, from 

 the expression singularly resemble, that it was something 

 more than in appearance, and this not very close. 



The tremolite, which derives its name from the vale of 

 Tremola near St. Gothard, one of the principal places in 

 which it is found, is a radiated mineral substance, the 

 thi-eads of which, most commonly of a shining white, are 

 united in sheaves or bundles. These bundles issue from 

 a common centre, and diverge around it, which gives them 

 the figure of a radiated star; and these centres being vari- 

 ous, they give different directions to the radii, which are 

 from half an inch in length to three inches or more. This 

 mineral substance is one of the most curious and pleasing 

 to the eye. It is sometimes intermixed with talc and calca- 

 reous spar ; that is with two substances, one of which is 

 vitrifiable but of difficult fusion, and the other calcinable: 

 a circumstance of itself sufficient to exclude the least re- 

 semblance between tremolite and the products of glass- 

 houses. And if we compare these products with theslender 

 and brilliant threads of the tremolite, each of which taken 

 separately has the form of a quadrilateral prism, we shall be 

 surprised, that they were ever compared with each other. 

 The tremolite is vitrifiable; but it is not and never was 

 vitrified. 



Let us now turn our attention to the crystallized sub- 

 stances included in lavas, to which the vitreous crystallites 

 have been compared. This comparison I am able to make 

 on a great number of pieces which I have collected from 

 burning and from extinct volcanoes. 



The lavas that include leucites, or white garnets, fre- 

 quently include likewise volcanic schoerls, augites or pyrox- 

 enes*, and chrysolites or olivines. Here are three spedies of 

 crystals, very distinct from each other both in figure and Co- 

 lour, contained in the same lava ; enveloped in the same 

 paste, which has no resemblance to either of them in nature, 

 colour, or chemical properties, as will soon appear. 



* I shall mention these in future by the name of pyroxene schoerls, 

 because the denomination of pyroxene does not belong to them exclu- 

 sively, all the substances contained in lavas being equally pjroxene, or 



•strangers to fire. 



. The 



