80 PROF. A. DAT7BREE ON INCRUSTATIONS BY THERMAL 



especially hornblende and augite, are found at times to be transformed 

 into a chloritic substance without change of their original crystalline 

 form. Such chlorite is therefore of similar origin to that found in 

 company with zeolites and carbonates in the geodes. The same is 

 true of the spilite of Drac, in which chlorite, green earth, and carbo- 

 nate of lime form a great part of the mass of the rock. The hyperite 

 amygdaloid of Nassau*, contains more than one fifth of its weight 

 of each of these minerals ; and the melaphyre of Lake Superior is 

 another example of a similar association, one specimen having been 

 found to contain 46 per cent, of delessite. 



Water up to 3 or 4 per cent, appears also to be a common con- 

 stituent of the mass of such rocks when containing carbonates. 

 Delesse found 3 per cent, in the melaphyres of Oberstein f. 



Demonstration of the Origin of Zeolites and associated Minerals in 

 Volcanic Rocks. 



The phenomena that we have previously demonstrated to be caused 

 by the action of thermal mineral-waters, has in former times been 

 developed to a very considerable extent in certain geological forma- 

 tions. The rocks in which zeolites are found, have been permeated 

 for a longer or shorter period by heated water, which, reacting upon 

 their constituents, has given rise gradually to various species of 

 minerals. 



As is the case at the present day, these eruptive rocks may have 

 been the seat of emanation of water at high temperature, which, 

 coming from great depths, penetrated the rocks at or near the sur- 

 face in the manner seen in the lavas of modern date. 



In addition to the above, it may be admitted that these essentially 

 permeable volcanic masses were readily accessible to rain and other 

 surface-water ; such an introduction of water in rocks of this vesicular 

 character is not only possible, but, so to speak, necessary, circulation 

 being effected by the innumerable cavities in the same way — in brief, 

 as by a system of drain-pipes. 



Water so infiltrated before the rock had completely cooled would 

 of necessity become rapidly heated ; and with every increment of 

 heat the solvent and decomposing power exerted on the surrounding 

 rock would be increased. But whether of superficial or deep-seated 

 origin, the action of the heated w*ater would be similar to that of the 

 thermal spring upon the masonry — the effect first being that of de- 

 composition, and subsequently of reconstruction with the formation 

 of new compounds. 



Zeolitic minerals may therefore be considered a kind of " extract " 

 of the rocks so subjected to continued lixiviation — a view that is 

 borne out by the more or less altered condition usually observed in 

 the rocks most abounding in this class of mineral — the alteration in 

 many cases having resulted in the production of an earthy state, 

 forming the so-called wacke of the older geologists ; and chemical 

 examination proves that in such cases the rocks are hydrated. 



* Ludwig, Section Gladbach, description, pp. 103-108. 

 t Ann. des Mines, 4 e ser. vol. xvi. p. 511. 



