Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)
Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján
Somewhat different is the rock type situated at a somewhat deeper level and farther off the Mulató Hill and Bólyatető Mountain. This type is more massive, containing less of biotite and tridymite and having a greyish-pink to light grey colour. Both kinds of rock are characterized by a thin banking near surface, becoming thicker in a depth of 3 to 5 metres. In the quarries, the usual rough columnar jointing of andésite is observed. The rock is traversed by a dense network of joints. The main joint strikes are 4 h —5 h , and ll h —12 h , with secondary but quite frequent strike orientations around 7 h and 2 h . In the lower section of Más Creek, globular weathering occurs. It is readily seen there to be a form due to the combination of banking and jointing. The main strike of striation is between 22 h and 2 h , with dip angles generally about 30 to 65°. The dip direction of striation is more or less coincident with the 3—4 hours' dip direction of banking. There is, however, a difference in the dip angle, which is gentler in the case of banking (about 15 to 20 degrees). This suggests that striation has formed simultaneously with the solidification of the rock. The upper part of striated andésite, the dip of which was identical with that of the banking, i. e. somewhat gentler, was eroded, the rock visible at present being the radical part of striated andésite. As related to the strike and dip values as described above, significant differences occur on Bólyatető, on the western part of Hosszú Hill and in the valley of Más Creek. However, even in the named areas, striation coincides with banking. There is an anticline-like upswell of biotitic pyroxene andésite on Bólyatető and on the western part of Hosszú Hill. This is proven by the following dip directions and dip angles, valid for banking as well as for striation : 24 h /65° on Bólyatető, 16 h /15° farther west, in the Gereg quarry, and 8 h /60° on Hosszú Hill. The opposite dip in the Más Creek valley, with dip directions between 12 h and 18 h to the banking, and 6 h to 24 h to striation, may be derived from the movements initiating endometavolcanitization rather than from variances in the position of the original lava. The mentioned movements have broken up to a smaller extent the already solidified rock and have locally dislocated it. Along the faults thus formed, metavolcanitization took place, and the joints later became filled with chalcedony, jasper and opal. Differential weathering has taken advantage of the hardness differences due to striation and has prepared out the hard striae (e. g. on Temple Hill, Gyöngyöstarján ; Mérőkő Creek valley), sometimes bringing about a sort of exfoliation (northern flank of Mulató Hill). This latter phenomenon is also observed elsewhere, especially where a steeper dip of the rock and a simultaneous occurrence of intense jointing have facilitated the circulation of ground water. The mean value of the mineralogical composition of typical striated biotitic pyroxene andésite is, as shown by the integrating stage measurement results on eight slides, as follows : feldspar phenocrysts 31,2 per cent, augite and hypersthene 6,1 per cent, biotite 3,0 per cent, matrix 59,2 per cent. Tha grain size distribution curves of biotitic pyroxene andésite exhibit three peaks. The first one is due to the feldspar laths in the matrix, to magnetite grains and to small grains of dark silicates. The second peak is more marked, being due to smaller porphyric ingredients (feldspar, hypersthene, augite, biotite, tridymite, chalcedony). The third peak is brought about by the big phenocrysts of feldspar and dark silicates. The differences between the individual slides consist in the varying amount of glass (4 to 20 per cent), in the variable
