SORBV STRUCTURE OF CRYSTALS. 477 



and portions of the surrounding pitchstone project right into them. 

 Such projecting portions of the glassy basis have often become en- 

 closed in the solid crystal, in precisely the same manner as the fluid- 

 cavities in crystals formed from solution in water, as shown by fig. 2. 

 Fig. 57 is a very good example of one of the larger of these glass- 

 cavities. The centre is full of glass, precisely like the general basis 

 of the pitchstone, except that the groups of green crystals are not so 

 large and well developed, whilst somewhat larger prisms than those 

 in the centre are attached to the sides, as if deposited during the 

 cooling of the glassy solvent. The accompanying bubble is no doubt 

 the effect of the contraction of the glass before it became solid. 

 Fig. 58 is an example of a smaller cavity, having all the green crystals 

 attached to the sides, It also contains several bubbles, which is a 

 fact very characteristic of glass-cavities, since it never occurs in fluid- 

 cavities, except under very peculiar circumstances, seldom met with. 

 A common kind of cavity is shown by fig. 62 ; but the very smallest 

 of all contain no green crystals, like fig. 60, corresponding therefore 

 to those very small fluid-cavities in which crystals have not been 

 deposited from a supersaturated solution. Fig. 61 is a case where 

 the bubble has been much distorted, and crystals project from the 

 sides quite into it, proving that the crystals were deposited before 

 the glass became solid. Besides these glass cavities, the felspar has 

 caught up small, colourless, contemporaneously-formed, prismatic 

 crystals, to which in some cases glass-cavities are attached. A very 

 excellent example of these is shown by fig. 59 ; and it is a striking 

 fact, that very nearly all the green, prismatic crystals have been 

 deposited on the included large crystal. The felspar also contains 

 bands of vapour- cavities, and it is near to them that cavities with 

 several bubbles occur ; but, at a distance from them, the glass- 

 cavities almost always, if not invariably, contain a bubble from ^th 

 to -g-th the diameter of the cavity. In some cases, however, there 

 are cavities like fig. 63, which do not contain the prismatic crystals 

 or a bubble, being more like stone-cavities. 



In the pitchstone are also some dark crystals, not visible except in 

 sections, which look extremely like augite. The glass- cavities in 

 these do not contain the green crystals ; and if the two minerals are 

 the same substance, this fact agrees with what takes place in crystals 

 formed from solution in water, the material being merely deposited 

 on the sides, and not as independent crystals. They, however, con- 

 tain bubbles, relatively of a smaller size than those in the felspar ; 

 whilst the glass-cavities in another mineral, the exact nature of which 

 I have not been able to determine, contain many green crystals, but 

 no bubble, as shown by fig. 64, probably owing to these minerals 

 contracting more than felspar in cooling from a high temperature. 



The analogy between these glass-cavities and fluid-cavities is there- 

 fore in many respects very striking ; and, as will be seen, their 

 peculiar characters can be most perfectly explained, if we suppose 

 that the glassy base, when in a state of fusion, acted like a solvent 

 liquid and dissolved various mineral substances, which were deposited 

 on cooling in precisely the same manner as crvstals are deposited on 



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