Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)

Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján

forty metres in the disclosure) seem to contradict the above considerations in some points. Around the top of the quarry wall there occurs typical biotitic pyroxene andésite, with the transitory type underneath and with dark hypersthene-augite andésite at the lowermost disclosed level. This suggests that the andésite occurring at present in the southern part of the volcanic body, on the surface, was solidified beneath a relatively thin sedimentary cover, the biotitic pyroxene andésite being a simultaneously formed, lateral facies of dark hypersthene-augite andésite. Transvaporisatory influence of the sedimentary cower was most intense at the top of the rock body, while farther off the assumed contact, towards the deeper parts, it gradually grew weaker and finally died out entirely. Consequently, while in the first case the assumption of a younger magma is necessary, which would have surged along the southern rim of the Mátra Mountains and would have differed in its chemical composition from the rest of the andésites of the area, we have according to the second assumption a striated biotitic pyroxene andésite facies formed as a consequence of differences in the circumstances of cooling. References : 1. M a u r i t z, B. : A Mátra hegység eruptív kőzetei. — Die Eruptivgesteine des Mátragebirges (Mat. és Term. tud. Közi., 30, 1909, p. 133—249). — 2. V i g h, Gy. : A Mátra déli aljának földtani viszonyai. — Geologische Beobachtungen any Rand des Alföld zwischen dem Zagyva-Fluss und dem Hidegvölgy von Bakta (Földt. Int. Evi Jel. 1933—35, 1939, p. 651—731). — 3. N o s z k y, J. sen. : A Mátra hegység geomorphológiai viszonyai (a debreceni Tisza István Tud. Társ. Honismertető Bizottságának kiadv., 3, 1926—27, pp. 148). — 4. S z á deczky-Kardoss, E. : A vulkáni hegységek kutatásának néhány alapkérdéséről (Földt. Közi., 88, 1958, p. 171—200). — 5. Szádeczky-Kardoss, E. : A magmás kőzetek és ércek képződési mélységének meghatározásáról (Magy. Tud. Akad. Műsz. Oszt. Közi., 20, 1957, p. 235—251). — 6. Szádeczky-Kardoss, E. & V i d a c s, A. : A Mátra hegység neogén vulkanizmusa (MTA Nemzetközi Geokémiai Konferencia előadásanyagából, 1959). Explanation of plates Plate I. Fig. 1. Light band of striated biotitic pyroxene andésite. At the middle of the lower margin of the photo there are tridymite crystals in a roof-tile arrangement. Plain light. 50 x. Fig. 2. Dark hand of striated biotitic pyroxene andésite. Porphyric hypersthenes are surrounded by broad limonite wreath. The matrix is rich in totally limonitized hypersthene needles. Plain light. 50 x. Fig. 3. Oxyandesite. The rock is in the stage of metavolcanitic opacitization. Its magnetite is decomposed, or altered into limonite, respectively, colouring most of the matrix. Plain light. 50 x. Fig. 4. Hydroandesite. In the less intensely altered specks pyroxenes are visible with still fresh magnetite inclusions, and also plagioclase feldspar, resisting alteration relatively well. Plain light. 50 x.