Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/392

296 " quartz-porphyrites." The " dacites," or quartz -andesites, are not of frequent occurrence in the Schemnitz district, but are seen at a few points, as the Hodritsch Thai near Schemnitz, the Kopa- nitzer Thal near Dilln, and the Berg Szitnya near Sz. Antal ; but in some other parts of Hungary and throughout Transylvania they have a very wide distribution.

The agglomerates, tuffs, and ash-beds which are associated and alternate with the sheets of lava are almost wholly made up of fragments of volcanic rock, sometimes mingled with portions of the subjacent sedimentaiy deposits. In some places the agglomerates consist of irregular masses of all dimensions, including blocks of prodigious size and weight, confusedly heaped together ; at others they pass into the most perfectly stratified tuffs and ashes. Such stratification in the ejected volcanic lapilli and sand must not, how- ever, in the absence of marine or freshwater organic remains, be ac- cepted as any proof of their having been accumulated beneath water; for, as we know from the example of many recent volcanos, ejected materials in falling through the air, or while being swept down the mountain-slopes in those torrents of mud so characteristic of great volcanic outbursts, may assume the most perfectly stratified and even finely laminated condition. The masses of volcanic agglomerate, as well as the rocks on which they rest, are frequently seen to be traversed by numerous dykes of lava.

The more finely divided tuffs and ashes are not unfrcquently found to contain numerous remains of plants. These have been most carefully studied, and their relations to the existing flora and to those of earlier geological periods very clearly determined by Ungcr, Yon Ettingshausen, Kovats, Andrae, Heer, and Stur. Erom the examination of these plant-remains wo are enabled to infer the conditions of climate, soil, and elevation which distinguished the old land on which these volcanic eruptions took place. Besides the plants, some remains of terrestrial mollusca, reptiles, and mammals have also been found imbedded in these volcanic tuffs.

That these agglomerates, tuffs, and ashes were formed by the ordinary explosive volcanic action, there is, I think, not the smallest room to doubt ; and that the volcano which they helped to build up was a subaerial one, there is also abundant, proof. That, as is so common among volcanic cones, numerous ponds, marshes, and even lakes of considerable size, were successively formed and filled up by the ejected materials and sediments derived from them, is also, I believe, rendered sufficiently clear from the lacustrine deposits with remains of freshwater mollusca and fishes which are so frequently found alternating with the agglomerates and lavas. But it is also evident that the ancient volcanos of Hungary, like so many existing ones, rose directly from the margin of tbe ocean, or rather of a more or less isolated inland sea, with semi- marine or brackish-water conditions. That into some of the depressions among the great volcanic masses of Schemnitz the sea must, from time to time, have had access, is proved by the marine or brackish-water shells found in the basins of Handlova and Rybnik,