Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 57. (Budapest 1965)

Csalogovits, I. J.: Geological and petrological conditions of the Szanda - Bercel area

taneously with the decomposition of the plagioclases, and the relative enrichment in oxidized iron and clay minerals take place. The various forms of the onset of hydroandesitization result in changes of composition that show various trends. It is characteristic of the changes in oxidation degrees and over-all iron content that this iron content of the contact hydroandesites is similar to that of fresh rock, while the iron content of hydrothermal hydroandesites and weathering hydroandesites is lower than that of fresh rock. The oxidation degree in original rock is 0.47 ; in endometavulcanites it is 1.21, compared with the 52.2 value of contact hydroandesite. A comparison of the values of over-all iron content and degree of oxidation with the values of fresh andésite makes possible a genetic discrimination between various types of hydroandesite. This does not apply to the above-mentioned rare opacitite endometavulcanite blocks, but the genetic classification of these presents no difficulties as they occur only inside the hydrothermal veins.