J. W, Judd — On Volcanos, 211 



An interesting circumstance with regard to the relations between 

 these great intrusive masses, which formed the centres of old Triassio 

 volcanos, and the sedimentary rocks by which they are surrounded, 

 was first pointed out by Leopold von Buch, and regarded by him as 

 being altogether inexplicable, as indeed, if his theory of " elevation- 

 craters " were true, it certainly would be. This is the constant 

 dipping of the stratified rocks around the different igneous centres, 

 not away from them, as we might at first be led to imagine would 

 be the case, but towards them. This fact is very clearly exemplified 

 at Predazzo, as the accompanying section, which has been derived 

 from one of those published by iiichthofen, will show ; and it was 

 also very clearly represented in the earliest sections of the district 

 given by von Buch himself. 



Predazzo . 



<* ff ff U V ^ ifd if e SF a> 



Section across the so-called " Crater" of Predazzo, illustrating the dip of the 

 stratified rocks towards the centre of eruption. 



a Eotzen porphyry (Permian), e Monzoni-syenite with " diabase" veins. 



h Lower Trias. \ f Tourmaline granite. 



c Middle Trias. ,' Sedimentary rocks. g Melaphyre. 



d Upper Trias. ) 



That these singular cup-shaped depressions of the strata around 

 each of the volcanic centres in the Southern Tyrol can have no 

 claim whatever to be called '' craters," in the ordinary sense in which 

 geologists use that term, we have already remarked. The true 

 explanation of the phenomena presented by them is to be found in 

 a fact, first pointed out by Darwin, but of which illustrations have 

 since been found in many widely separated volcanic districts — namely^ 

 that the ejection of volcanic materials at any point is liable to be 

 followed by subsidence of the rocks around it. I have elsewhere 

 shown how very frequently this kind of action takes place, and what 

 beautiful illustrations we have of it even in our own islands. From 

 my study of the rocks of the Southern Tyrol it appears to me to be 

 perfectly clear that, after the cessation of volcanic activity at the 

 numerous small centres of eruption, the central mass must, in almost 

 every instance, have subsided before the denuded ruins of these old 

 volcanos were buried under the later deposits. 



Perhaps, however, the most interesting circumstance of all con- 

 nected with these old volcanic outbursts is the insight which they 

 afford as to the mode of origin of those beautifully crystallized 

 minerals which are so frequently ejected from the craters of active 

 volcanos, like Vesuvius, or are found among the agglomerates and 

 tuffs of extinct ones like the Laacher See. There is probably no 

 locality in the whole world which has yielded so large a number of 



