236 THE ERUPTION OF VESUVIUS IN APRIL, 1906. 



copper oxide (teuorite). These fumaroles are neutral so far as they 

 are really dry. AVhen the temperature permits the appearance of 

 water vapor, they become acid (hydrochloric acid, then sulphurous 

 acid) ; their sublimates are then colored yellow, red, or green by 

 metallic chlorides and sulphates. Later come the fumaroles with 

 ammonium chloride ; then those characterized by deposits of sulphur, 

 with gaseous products rich in hydrogen sulphide; and last of all are 

 the fumaroles containing carburetted hydrogen and carbonic acid. 



It is well understood that these divisions are somewdiat arbitrarily 

 established and that the products of one type are frequently trans- 

 formed by the gases of the type that follows. It has apparently 

 been w^ell established by Palmieri that at Vesuvius cuprous chloride 

 of the acid fumaroles results from the attack of the hydrochloric acid 

 of the cooler type upon the tenorite of the dry fumaroles; that 

 hematite once formed can be changed into ferrous chloride; that 

 calcium sulphate, common in the lower types, can form at the expense 

 of the calcium chloride. 



Finally, beside the products of sublimation brought from a greater 

 or less depth, are others resulting from the attack upon the walls of 

 the fumaroles by acid vapors. This is common in the fumaroles with 

 chlorides and sulphates in which minerals are formed at a tempera- 

 ture below that demanded by the true sublimates. 



Lavas in contact with fumaroles of high temperature are strikingly 

 fresh, whereas the same rocks are profoundly altered when exposed 

 to the same A^apors at a temperature low enough to permit the con- 

 densation of water vapors. 



FUMAROLES OF THE LAVA. 



I was unable to observe the dry fumaroles in action, but their 

 trace was found at Boscotrecase as white, green, or yellowish con- 

 cretions serving as a support for the crystallized ammonium chloride. 

 These concretions contained also a large proportion of the chlorides 

 of potassium and sodium, in some cases with a little iron and alu- 

 minum and traces of lead and copper. 



The true acid fumaroles, rich in ferrous chloride, are only found 

 toward the source of the flows and, as I shall show farther on, they 

 apparently mark the place from which the lava issued. This is 

 generally true of Vesuvius except in the greatest flows. Hematite, 

 also resulting from the reaction at a high temperature of water vapor 

 upon ferrous chloride, is rarely found in the flows, but is common 

 at their points of emergence and in the crater. 



The fumaroles at a temperature below 400° C, with a weak acid 

 reaction, are very abundant in the terminal portions of the flow of 

 May 8. They furnish magnificent geodes containing crystals of 

 ammonium chloride, generally colorless, but in some cases with a 



