536 THE POPULAR SCIENCE MONTHLY. 



have been gathered at Santorin in the interior of tubular cavities like 

 fulgurites, which have evidently been traversed by vapors at a very 

 high temperature. The deposited crystals are of anorthite, sphene, 

 and pyroxene under the forms of fassa'ite and augite. 



Other crystallized silicates are met occasionally in the lavas. They 

 arise from the transformation of the calcareous masses which are taken 

 from the subsoil in the region and carried out in the lavas, and consist 

 chiefly of anorthite, fassa'ite, the melanite garnet, and Wollastonite. 

 The quartz and mica-schist, which, like the limestone, appear as in- 

 closed lumps in the lava of Santorin, do not seem to have suffered any 

 action from the matter surrounding them, notwithstanding the high 

 temperature it possessed. 



The mineralogical study of the lavas of 1866 offered great difficul- 

 ties, on account of the small size and strong adherence of the mineral 

 integrants. It was not practicable to extract the crystals by picking 

 them out, and M. Fouque was obliged to seek new processes. One, 

 founded on the employment of a powerful electro-magnet to draw out 

 the ferruginous minerals, permitted the isolation of the feldspar ; and 

 other minerals, harder to deal with, were separated by the use of con- 

 centrated hydrofluoric acid. 



The crystallization of minerals in the recent lava took place in two 

 stages. In the first stage were develojDed crystals which frequently 

 attained the length of about a fiftieth of an inch, and of which the 

 other dimensions exceeded a tenth of that size ; and, in the other stage, 

 crystals of notably small dimensions were produced. The latter crys- 

 tals, or microliths, unite the others and follow their contour. Before 

 the microscope was applied to the study of the rocks, only the larger 

 crystals were recognized, and the matter which in volcanic rocks 

 envelops crystals large enough to be perceived through the glass was 

 supposed to be wholly uncrystalline. In this matter, however, the 

 microliths abound in immense quantities. The discovery of them has 

 been one of the most signal triumphs of microscopic micrography. 



The minerals observed in the microliths of the general lava of 1866 

 are feldspar and titaniferous oxide of iron. The predominant feldspar 

 in large crystals is the labrador, but anorthite, and some oligoclase and 

 sanidine are also found in the same condition. The microlithic feldspar 

 is albite with a considerable proportion of oligoclase, and the whole is 

 cemented with a vitreous matter which represents the residue of the 

 crystallization, the part of the rock to which the latter owed its fluidity, 

 at the moment when the minerals which had made a portion of it were 

 already crystallized. This amorphous matter is of a composition simi- 

 lar to that of albite, but a little richer in silica and potassa ; and it is 

 curious that a substance of such a composition should have constituted 

 the part of the rock which remained longest melted. 



The order of crystallization of the minerals was as follows : 1. Mag- 

 netic oxide of iron in large crystals ; 2. Apatite ; 3. Silicated magne- 



