Lazarro Spallanzani (1729-1799), Italian abbot, Professor of Natural History at Padua,was one of the first to use experimental methods in the study of rocks.

The first time I ventured to explore the bottom of~the crater of VuIcano, I only found some fragments of this prismatic lava: but when I repeated my visits, and had divested myself of the fear I at first felt, and more carefully examined this dreary bottom, I was enabled to complete my discovery by ascertaining the origin of these prismatic, or, as some may choose to call them, these basaltiform lavas. For raising my eyes to that part of the crater which was over my head, and facing the northeast; I perceived a large stratum of lava, almost perpendicular, divided lengthwise into complete prisms, some of which were continued with the lava and made one body with it...

The production of these basaltiform lavas, which, from their situation, and their forming a whole with the lava, no one can doubt derive their origin from fire, may, I conceive, be thus explained. In former times an effervescence took place in the melted lava in the crater, which, after having swelled, and perhaps overflowed its edges. Slowly sunk in the cavity of the crater, from the

diminution) of the fire, and the impellent elastic substances, while a portion of the lava attaching itself to the internal sides and hastily cooled by the atmospheric air, contracted, and divided into regular parts, such as are the forms of the hexagon prisms above mentioned.

In various parts of this work mention has been made of the gases of volcanos. It has been shown that, by their elasticity, stonysubstances fused in the fire are rarefied, inflated, and become cellular; as is proved by the great number of lavas, glasses and

enamels. We have seen that, by the violence of these gases, the liquefied matters are hastily raised from the bottom of thecraters to the top, filling their whole internal capacity, and flowing over their sides; since, by the action of the same gases, we frequently observe similar phenomena in the furnace.

I shall now proceed to enquire what part this aeriform vapour acts in the eruptions of volcanos. Where it exists in the depths of a volcanic crater, abundantly mixed with a liquid lava violently urged by subterranean conflagrations, I can easily conceive that by its energetic force it may raise the lava to the top of the crater, and compel it to Row over the sides, and form a current.

It is likewise probable that this elastic vapour, when collected in a large quantity, if it finds under the earth any impenetrable obstacle, produces local earthquakes, and subterraneous thunders and roarings; bursting open the sides of the lava and forcing out the lava. We have an example of this, if I may so speak, in miniature, in the two matrasses* broken by this fluid from its exuberance and the resistance It met with.

* Containers to hold crushed material melted by Spallanzani in his experiments.--EDITORS.

Nicolas Desmarest (1725-1805), a French physician, geologist, and mineralogist, whose writings show exceptional clarity of thought. His appeal to observable facts was well calculated to project the light of reason into the heat of unrestrained argument concerning the real nature of extruded igneous rocks at a time when many leading scientists were inclined to the theory of their aqueous origin.

I have seen many of these isolated masses of basalt, and I admit that if I had been limited to these masses in my observations, I would not have been able to decide that basalt was a compact lava. It is only in going from the simple to the composite that I put myself in a position to establish this truth and to generalize on its application. The infinitely varied results of the operations of fire which are exposed in Auvergne presented the most favorable circumstances in some places and elsewhere displayed the greatest changes in the conditions. I paid attention first to the flows in which the prismatic basalt occupied the center and the edges and in which I have recognized an uninterrupted continuity from their most distant extremities to the open mouth of a volcano. They appeared before me accompanied at the same time by all the phenomena of porous lavas, scoriae, pumices, and baked earth, and lying for the most part on unchanged bases over which the melted materials progressed. Such are the circumstances which guided me in the beginning of my observations. Once enlightened on the primitive state of the phenomena, I have reasoned that the alterations undergone in the disposition of the masses of basalt in certain districts could not invalidate that which has been recognized and well proven in others.

Thus where I have found isolated buttes, composed of black stone, which had the same grain, the same brilliant and glassy

I points, the same prismatic or rounded form as basalt, I could not persuade myself that if the first that I had observed in the recognized and continuous flows were lava, the latter would not be a similar product of fire.

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I believed it necessary, elsewhere, to describe the first alterations that wiped away the craters and the scoriae, and finally the changes which have taken place in the sites which these flows occupied in the bottom of the valleys. All these circumstances seemed to me to bespeak variations which have practically the same sequence. As soon as craters have their edges blunted or widened, or even have begun to fill up; as soon as the scoriae begin to be reduced to a pulverized earthy substance, then the flows which have emerged from these centers of eruption and which are stripped of their scoriae, no longer occupy the bottom of the valleys. They are situated halfway up the hill, the valley being deepened since the flows were established on its former floor. The torrents which skirt them or traverse them, moreover, have lowered part of their mass by jagged cuts which are strewn with falls and cascades.

As it is only by a very long series of centuries that all these forms and all these circumstances have changed, it is easy to show the causes as well as the progress of the variations. Observation first informed me that the scoriae and spongy substances experience a very noticeable comminution and are finally reduced, in a rather short space of time, to pulverized earths. In addition it has shown me the water of rains and melted snows continually displacing these mobile materials. As a result of this double work of water, the edges of the craters, formed mainly of scoriae, must be blunted; then these mouths fill by imperceptible gradations, and finally disappear entirely.

It is in this epoch that the horizontal beds formed in the first and most ancient epoch were cut by valleys; that the craters open during the middle epoch were destroyed or filled up; . . . that the different parts of the flows themselves, established on the surface of the horizontal beds, have been separated by cuts which have gradually become valleys of the first order. It would appear that during the most recent epoch the valleys which separate the portions of the same flow must have increased and become deeper at the same time that the destruction of the craters and the I comminution of the scoriae took place.