146 Scientific Proceedings, Royal Dublin Society. 



whole, a highly vitreous mass, contains large crystals of sanidine 

 scattered through its mass. 



Triclinic felspars. — Other felspars, such as labradorite, 1 were 

 produced by a recuit of some days ; but large crystals, such as 

 are met with in the Etna lavas, do not so far seem to have been 

 obtained. Such a result is easily explicable, when we are informed 

 that to produce a microlith some days are required, whereas we 

 know that even after the expulsion of the lavas of Etna many 

 months or years are requisite for their cooling, so that recuit may 

 be prolonged far beyond the limits within which we can experi- 

 ment. If a large stream of lava, such as issued from Etna in 

 1669, be examined, it will be found that even that which was 

 cooled immediately contains crystals of labradorite, which indi- 

 cate the plutonic origin of that mineral, or that the magma had 

 been undergoing a prolonged recuit in the upper part of the chim- 

 ney. Specimens taken from the centre of some of the thicker 

 parts of the stream far from its source, and which must have been 

 long in cooling, we find the crystals of that felspar therein con- 

 tained have attained greater dimensions, thereby indicating that 

 under favourable circumstances this mineral may undergo further 

 growth after extrusion of the lava. A similar occurrence I have 

 noticed at Vesuvius. At Cisterna is a gigantic lava stream that 

 is known to be more than half a kilometer broad, and its depth 

 beneath is unknown, although it is quarried to a depth of twenty 

 meters, at a distance of more than ten kilometers from the original 

 eruptive axis of the mountain. Now, of all the lavas of Monte 

 Somma this is the most extremely crystalline one, all its constituents 

 being of very large size, and practically all the amorphous paste has 

 passed into a crystalline condition. So far as is known, this is the 

 greatest outpour that ever occurred from this mountain ; and, no 

 doubt, in consequence of its enormous thickness, being unrivalled 

 by any modern stream, a very long time must have been occupied 

 in its cooling, conditions highly favourable for the production of 

 a coarse structure. When small streams dribble from the crater 

 after prolonged stromboliau action, the structure also is often 

 very coarse, as the part of the lava column in the chimney has 

 been gradually loosing its heat. Anyone who visits Monte 



1 Fouque and Levy, Comptes rendus, 1878, t. lsxxvii., p. 700. 



