Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 57. (Budapest 1965)
Csalogovits, I. J.: Geological and petrological conditions of the Szanda - Bercel area
ted to Mt. Bercel' s symmetrical cover which is intact also at the edges — the andésite mass adjoining horizontally the slip volcano openings of Szanda on a higher level is a denuded bottom of a large cover that extended originally over the cover of Mt. Bercel. This former assumption is supported by the doleritic texture and higher degree of crystallization of Bercel andésite, the latter itself indicating the existence of a cover that slowed down cooling. B) Chemical analysis of andésite types. A thorough pétrographie analysis of the pyroxene-andesites of the Szanda area was made by F. SCHAFARZIK, A. VEXDL and R. BEICHERT. These authors have named the vulcanite types found in this area augite-microlitic augite-andesite, and augite-microlitic augite-hypersthene-andesite. The genetic circumstances and the interrelations of these types have not been described thus no differentiation phenomena were mentioned either. Knowing the chronological order of melt effusion — according to which the parts at the edges come first, the parts nearer to the openings second, followed by lava stuck in the openings in the course of effusion — the analysis of samples taken from suitable places supplies information also of the temporal process of differentiation. Based on the scheme of cover formation as outlined in the foregoing, we have compared the succession of lava effusions on an analytical basis. According to the picture resulting from this comparison, the chemical composition changes temporally within eruption, but the order of magnitude of these changes is small (53,7%—56,5% Si0 2 ) and shows no uniform tendency. Concerning the lava effused later on it may be stated that it usually shows a higher degree of silification than the earlier one. But the changes in the degree of silification show local fluctuations and this may be a consequence of the non-homogenous distributions of the intratelluric minerals — to be discussed later — in addition to the original inhomogeneity of molten lava. C) Microscopical investigation of andésite types occurring in the area The investigated samples were taken from the Szanda-Péterhegy roof and base in the Szanda dike-slip volcano system, the material of the Délkut dike forming the bottom of Szanda, roof and base of the cover of Mt. Bercel, and the rocks of the Istenhegy (Bercel) stone pit in the southern range. The investigation was extended to include also the hydro-andesites that were formed hydrbthermally, under contact effect and by weathering. 1. The rocks of the Szanda range. a) Bytownite-phenoandesite (Szanda „roof andésite", Péterhegy, SW side) This rock is macroscopically fresh, coarse-grained, with 4— 5 mm plagioclase porphyries. The thin section shows that the bytownite porphyries were formed in two generations. The larger ones fluctuate between 2 and 5 mm and contain oriented, small augite-spicule inclusions. The ratio of maximum and minimum grain diameter is 3.6. The composition of the 2nd generation is a value between bytownite and labradorite. The number of inclusions in these is lower. The average size is 1—1.5
