462 Royal Institute of Frazce. : 
the peculiar problem long debated upon: If the crystals in thé 
lavas have been taken up completely formed from the bowels of 
the earth, and enveloped by them, or if they are formed after- 
wards in t! 1eir vacant spaces or cells; or, finally, if they have 
crystallized at the same moment when the rest of their mass 
was hardened: and he gives us to understand that it is the latter 
opinion which he adopts. 
He terminates his curious investigation by a methodical enu- 
meration of basalts and other products of volcanoes arranged 
according to their materials of aggregation, and under the ban- 
ners of the two substances which therein predominate, feldspar 
and pyroxene. 
This mysterious nature of velcanoes,these immense foci of heat; 
far removed from all the conditions which keep up heat atthe _ 
surface of the earth, will be still a long time one of the great 
objects of the curiosity of natural philosophers, and will excite 
their efforts so long as any hopes of success remain, A young 
mineralogist as zealous as he is learned, M. Mesnard de la | 
Groye, having had occasion in 1812 and 1813 te observe se= 
veral of the phenomena of Vesuvius, drew up a journal of them 
with great accuracy, intermixed witly many original suppositions 
and ideas 
Since the enormous diminution which the cone of the voleano 
underwent in 1794, when it sunk more than 400 feet, all the 
eruptions have taken place from its summit ; which seems to have 
prevented them from being so abundant and so destructive as 
those which issued from its sides. The bottem of the crater. 
rose, and it is not unlikely that it will be filled. 
_ The rivers of lava are the less abundant if a great quantity of 
scoriz and small stones are thrown out during the eruption. 
The whole cone is covered with those small stones, which are 
soon changed by the acid vapours, and assume those lively and) | 
variegated colours which make them look like bunches of flowers 
at a distance, and which have inclined naturalists to suppose 
that the crater is filled with sulphur; which is so far from being 
true, that it is even very rare that sulphurous vapours are per- 
ceived in it: on the contrary, there rise strong and continual ex- 
halations of muriatic acid, and sea salt is every where concreted 
throughout. 
M. Mesnard de la Groye thence takes occasion to divide vol- 
eanoes into two classes; those in which sulphur performs an 
essential part, and those in which the muriatic acid prevails. It 
is among the latter that he classes Vesuvius. 
He also notices the continual smoke which rises from the 
rivers of lava, and which annoutice great humidity. This smoke 
is in fact purely aqueous. No flaines are-seen, but sands and 
burnt 
eae fe seeciniiaiieaianelil 
