when slightly moistened. It absorbs water greedily, and with a kind of hissing noise; but is not reducible to a lubricious paste as clay is. But the internal part of this lava, besides being of a grey colour, is three-fourths heavier, and in its compactness, and its grain, approaches to that species of calcareous earth, called calcareus æquabilis, though, in fact, it only resembles it in appearance, not being reduced to calx by fire, nor dissolved by acids. In this lava, the sulphure of iron is not found in cubes or globes, but in thin lameliæ; and is dispersed throughout its whole substance, especially in certain parts, where the colour of the stone inclines to black, and has a greater consistency. No sign of this mineral appears in the white decompounded lava, probably because it was destroyed gradually, in proportion as the decomposition took place.

IV. This lava is much heavier than the three preceding; which, no doubt, arises from the greater abundance of sulphure of iron that it contains. The shining particles of this mineral are principally to be seen in its vacuities (of which, however, it has not many). They are polyhedrous, but the number of their faces is not constant. When exposed to the fire, it loses its brassy colour, burns with a thin blue flame, and emits a strong smell of sulphur. The lava which contains it, and which is of a livid grey colour, is in some situations so soft that it may be scratched with the nail, but in others much harder, and some of it will give sparks with steel. In this lava, the base of which appeared to me to be horn-stone, we find crystallized feltspars, but decomposed, though less so than the lava in which they are inclosed.

V. Around the extensive plain of Solfatara, we observe, in several places, a circular ridge of steep rocks, which once formed the upper sides of this enormous crater. The rain water, descending this declivity, over the decomposed lava, carries down with it the more minute parts to the lower grounds, where various concretions are produced, especially those stalactites which are commonly called. oolites, or pisolites *. But of these stalactites we shall speak hereafter. Here we shall only notice, that this water, in its descent, carries down with it small pieces of decomposed lava, and that, in

* Varieties of the Tophus oolithus of the Syst. Nat, or the Compact Lime. stone of Kirwan,

some places, many of these pieces are found united, and bound together by a crust of sulphure of iron. It is black where it is exposed to the immediate action of the air, but, in the fractures of a shining appearance, though the colour inclines more to a lead-colour than to yellow. Its s ructure is scaly. The sulphures of iron which have before been mentioned give fire with steel; but this does not, from want of sufficient hardness. It abounds with sulphur; since, being exposed to the flame of the blow-pipe, it visibly melts, and, the activity of the fire being increased, a blue flame arises, which continues till the crust is consumed, nothing remaining but a very small quantity of a white palverous earth, which is no other than a portion of decomposed lava, that had been united with this sulphur.

With this sulphur, the presence of which is extremely manifest from its strong smell, is also united arsenic; as sufficiently appears from the white fumes which arise from the combustion of the sulphure of iron, and which emit a very sensible odour of garlic.

These are the volcanic matters which, at Solfatara, abound more or less with sulphures of iron. But whence is their origin? It is well known they are formed by the combination of sulphur with iron. With the former this volcano abounds, whence it obtained the name of Solfatara; and, as the latter is almost always found mixed with volcanic productions, which commonly derive from it their varying colours, we have thus the two proximate principles of sulphure of iron. But is their combination effected by the dry, or, as is more probable, by the humid way? I find it difficult to conceive how it can take place by the first method, on account of the speedy dissipation of the sulphur sublimed by fire, which must prevent its uniting with the iron to form these sulphures. It appears to me more probable that they have been formed by the action of water, which having penetrated the lava, the sulphur, dissolving in the fluid, has combined with the iron. But as such solutions of sulphur in water seldom take place, as Bergman has observed, we rarely find sulphures of iron, in volcanized countries, notwithstanding the existence of these two minerals.

But let us continue the description of the productions of this celebrated place, the greater part of which are decomposed Javas; though this decomposition, notwithstanding it has been noticed by several writers, has not, to my knowledge, been examined by any one with requisite care and attention.

VI. This lava is coloured on the upper part with a covering of yellow oxyde of iron, under which is a white decomposed stratum, to which corresponds another lower one of a cinereous colour, where the lava is much less changed. These two strata form a very strong contrast. The white may be cut with a knife, in some places more easily and in some less; adheres to the tongue, does not give sparks with steel, feels soft to the wet finger passed over it, has consider able lightness, and being struck with a hammer, gives a dull sound, like earth moderately hardened. On the contrary the cinereous stratum sounds, when struck with a hammer, like a hard stone, of which it also has the weight; is rough to the touch, scarcely at all adheres to the tongue, gives fire with steel, and cannot be cut with the knife. The white stratum in some places, is an inch thick, and in others more, but there are likewise places where it is only a few lines in thickness. The white stratum, in general, changes insensibly into the cinereous, but in some places the separation is suddeu and abrupt.

The feltspars in this lava (for of these it is full) are prisms; the largest of which are ten lines in length, and the smallest the sixth of a line. In the cinereous stratum, notwithstanding a beginning de composition may be perceived, the feltspars are unimpaired. On the contrary, in the more decomposed stratum, I mean the white, their decomposition is very apparent; they have all lost their transparency, though many of them still retain their splendour. Others have acquired a resemblance to a sulphate of lime that has remained some time in the fire; to which they might likewise be compared in softness, had they a little less consistence. Some of them are infixed in that part of the lava, the colour of which is between the cinereous and white, and here we find them less changed than in the stratum which is entirely white. Others have one part of them in the white, and the other in the cinereous stratum; in which case we find the part fixed in the latter stratum to have suffered nothing, but that in the former considerably. In short, from the inspection of this lava, it is manifest, that, in proportion as the nature of it is changed, the feltspars it contains undergo a change, except when the principle producing the alteration is unable to affect them. Be sides these feltspars, we find incorporated with the lava a number of very small and almost invisible black shoerls, which are not distin→

guishable where the lava is white; less, perhaps, because they do not exist, than because they have lost their colour, in consequence of the decomposition.

This lava, which is of a margaceous base, does not liquify in the furnace, when its decomposition is considerable; but other parts of it, which have been less decomposed, are reduced to a kind of frit.

VII. Solfatara, perhaps, does not afford a lava more compact, hard, heavy, or of finer grain than this. Its composition is siliceous, its colour grey, it gives sparks strongly with steel, and, at the distance of two lines, attracts the magnetic needle. Its base is of the petrosilex, and it contains within it different feltspars and shoerls; but some of the latter have been melted by the fire, as appears from the bubbles or speckles occasioned by the liquifaction. This lava is covered with a very white crust, nearly an inch thick, produced by the decomposition it has undergone. The effects of the furnace on this lava, are nearly the same with those on the lava No. VI,

VIII. This lava is entirely decomposed. On the surface, and for some depth, it is white, and almost pulverous; but in the internal part the white colour changes into a reddish blue, and acquires a degree of hardness; though not too great to be cut with a knife. The feltspars, in which it abounds, have suffered different degrees of decomposition. Some of them, besides being calcined, attach strongly to the tongue. Others, when viewed with a common lens, appear full of filaments; but when examined with a deeper magnifier, these filaments appear to be no other than extremely thin, striated, and very friable lamina. This production is infusible in the furnace.

IX. The feltspars, in this lava, occupy more than one-third of its mass. They are in shape flat prisms, and, except having somewhat less hardness, retain all the qualities which characterize the species of stone to which they belong. There are also a number of shoerls, which, from their extreme minuteness, appear like points, but are easily distinguishable, by their black colour, from the lava, which is whitish, and has greater consistence than that of No. VIH. It is likewise heavier; to which the quantity of feltspars but little changed, which it contains, undoubtedly contributes.

X. The shoerls which make so great a part of the other kinds of lava, are found so strongly adherent to them, that we usually can only separate them in fragments. The present lava, in this respect,

offers an exception which may be considered as recommendatory of it. It has acquired so great a degree of softness by its decompo-, sition, that the numerous shoerls it contains may be detached from it entire. They are hexagonal prisms, truncated perpendicular to their axes, the faces of which are slightly striated lengthwise, and their colour is a yellowish black.

In this lava, the base of which appeared to me of horn-stone, another more remarkable peculiarity presents itself. On breaking it, the fractures discover a number of small caverus jewelled, if I may employ the term, with a multitude of extremely minute shoerls of different colours, some green, some yellow, others of a dark chesnut, but all similar, being hexagonal prisms, with rhomboidal faces, and each terminating in a dihedrous pyramid. Their angles are regular, their faces shining, and in part transparent. They sometimes form geodes in the body of the lava. To examine them a lens is necessary, and a good maguifier clearly to perceive other shoerls still more minute, These are infixed in the small cavities before mentioned, and though they are extended to a considerable length in front of the others before described, are so minute and numerous that a single cavity will contain a hundred of them. Every one of both these kind of shoerls has one extremity fixed in the lava, and the other in the air, and all together appear like a wood in miniature. I was at first in doubt whether I should consider them as shoerls or volcanic glass, as more than one instance has been known of such glass reduced to a capillary minuteness within lava. But the latter appeared to me improbable, because after all the observations that have hitherto been made, we are not yet certain that any volcanic glass has been found crystallized; for, with respect to the pretended crystallization of some glasses in Iceland, we have not facts which demonstrate it incontrovertibly. On the other hand, the minute corpuscles I have described, if not all, at least those which from their larger size are more descernible by the eye, have a prismatic figure, and analogy must induce us to conclude the same of the rest.

I incline to believe these infinitesimal crystallizations produced, after the cooling of the lava within the cavity in which they are found from extremely subtle shoerlaceous sediments by the filtration of water.