Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)
Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján
tion is labradoric, Ab sn An w , to bytownitic, Ab 2;i —An 77 . Most of feldspar is polylamellar plagioclase, containing frequent inclusions of rock glass, scarcer ones of matrix and augite and quite infrequent ones of hypersthene. In the neighbourhood of the pseudotuffaceous zone, feldspar is intensely altered, chloritized. Among the dark ingredients, the proportion of augite to hypersthene is rather variable. The number and width of the grain size categories occupied by these minerals is greater than in the case of biotitic andésite (90 to 120 ft, 240 to 340 fi and 500 to 600 ft respectively). Their inclusions consist mostly of feldspar. On alteration, hypersthene is bastitized, augite is chloritized. Magnetite presents itself in hexagonal cross sections of 60 to 80 ft, diameter, frequently with limonitized rims. The feldspar microliths of the matrix belong to the 6 to 12 fi and 30 to 100 //. size groups, forming 27 and 13 per cent of the matrix, respectively. They are generally arranged in fluidal patterns around the porphyric ingredients. Magnetite occurs in aggregates of grains of 5 to 10 and 10 to 30 fi size, limonitized in the more intensely weathered parts of the rock. It forms 18 per cent of the matrix. The amount of rock glass is quite significant, 12 per cent on the average. The crystal germs of the dark ingredients have an abundance of about 50 per cent. The rock variety just described occurs north of the area of extension of biotitic pyroxene andésite and pseudotuff, in a contiguous mass of great thickness. According to its macroscopic appearance, as well as to its great glass and small phenocryst content, it is the lowermost member of the youngest andésite effusion (mátraite). Its general dip direction is 8 to 10 h , with a dip angle of 10 to 15 degrees. Joints are quite frequent, with main strikes of 3 and 9 h . In the northern part of the area, on the southeastern flank of Havas Hill and around the ,,Kisdomb" keeper's house, at Height 309, the rock is interwoven by a network of quartz veins of 150 fi average diameter.The andésite of Kőmorzsástető ( = Crumbling Rock Peak) is falling apart into a rough gravel, covering an extended area on the surface. At the northeastern apex of the area, in the direction of Bánya Hill, a kaolinitic andésite variety becomes predominating. Endom eta volcanic products The phenomenon of endometavolcanite formation, recognized by Academician Szádeczky-Kardoss, can be studied to advantage in our area. According to Szádeczky-Kardoss's statement concerning the ore formations at Gyöngyösoroszi, the endometavolcanite zone of that environment was developed by leaching, resulting in the formation of kaolinite (4). In the Gyüngyöstarján area, there also occurred a metavolcanic process, if only to a smaller extent, with leaching resulting in the formation of clay minerals and varieties of silica. In the Csege Creek valley, there is a natural disclosure of about 30 metres length and E —W strike, exhibiting a collection of the most important metavolcanitic products of the area. On the eastern flank of the gorge there is the fresh variety of dark hypersthene-augite andésite, with its banking dipping towards 8 to 10 h , showing spheroidal exfoliation of the blocks defined by the joints. The fissures, becoming of ever greater width on progressing alteration, are filled with the clayey alteration products of decomposed andésite, the clay being traversed by siliceous veins. Towards the north, in a length of 100 to 150 metres, there
