398 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



frequently in lines parallel to the outer walls of the crystal ; more- 

 over the banded structure often seen in these crystals, either with or 

 without the aid of polarized light, in this case conforms more or less 

 to the lines of cavities ; but since the banded structure seems often 

 to be strongly developed without the presence of any visible lines of 

 cavities, there may be no direct authority for connecting the two 

 together. Still the very marked bands occurring in some pseudo- 

 morphs, as in figs. 1 and 2, certainly suggest a facility of access of 

 the replacing mineral along certain lines corresponding to the 

 original planes of growth. 



4. Leucitic Basalt. Albano, near Borne. 



The rock here described belongs to an old lava-flow of the Alban 

 Mount. It is very compact, and is quarried for road-mending beside 

 the bridge spanning a stream near Albano Railway-station, and be- 

 tween it and the town of Albano. The leucite crystals are generally 

 small ; and while there is more of the general base between them than 

 is the case in the rock represented in fig. 3, there is far less than in 

 the last-described specimen. One general character of the leucite 

 here is its semireticulated appearance, an irregular network more 

 or less covering the crystal-sections. In a good many cases the 

 circular arrangement of the stone-cavities, rather near the circum- 

 ference of some of the crystals, is evident, though it is perhaps 

 not quite so general as in the 1631 lava of Vesuvius. Some of the 

 crystals present the beautifully striated structure, in polarized light, 

 referred to in describing the last example ; but in one case, where 

 there is a collection of leucite crystals, iviiJiout any intervening base, 

 this structure is seen to great perfection (fig. 32), different series of 

 most beautifully parallel lines meeting one another at various angles, 

 frequently right angles. 



Zirkel remarks * that since the discovery of the tetragonal crys- 

 tallization of leucite, light is thrown upon the peculiar optical rela- 

 tions of the mineral, which were scarcely to be explained on the 

 former belief of its crystallizing in the cubical system. But previous 

 to this, Biot and Des Cloiseaux had observed that leucite, under 

 polarized light, did not behave like a crystal belonging to the 

 " regular " system, f 



In this specimen the augite is very plentiful, both in large 

 crystals and in very small particles, helping to form the general 

 base. Some large augite crystals have caught up the leucitic mat- 

 ter while this was yet in a viscid condition: and there are cases 

 in which the leucite has most clearly formed around the previously 

 solidified augite. As in the former examples, the augite crystals 

 frequently contain magnetite grains. 



To this general account of a few leucitic lavas I add two analyses 



* Mikroskopisolie Beschaffenheit, p. 152. 



( 1" Des Cloiseaux made an elaborate examination of a leucite crystal, "with a 

 view to learn something about the polarization stria; : see " Noxivelles recherches 

 sur les proprietes optiques," &c. (Paris, 1867), pp. 3-5, Inst. Imp. dela France, 

 tome xviii. 



