III.—A Petrological Study of the Arthur's Seat Volcano

Abstract

SynopsisAethur's Seat is the remains of a dissected Lower Carboniferous volcano, from which were erupted a series of basaltic rocks representative of the province to which it belongs. The rocks occur as lavas, intrusions and tuffs, and include basalts of Dalmeny, Jedburgh, Dunsapie, Craiglockhart and Markle types, in addition to mugearite. For convenience the basalts of Dunsapie type have been subdivided into Normal and Feldspathic varieties.The volcano has been remapped; some minor departures from the six-inch Geological Survey map are recorded and certain changes in the numbering and classification of the lavas have been made.The petrographic characters of the various volcanic rocks are described in detail, with modal and chemical analyses. The composition of the parental magma of the volcano was probably very similar to that of Normal Dunsapie basalt. Markle and Craiglockhart types were produced as complementary differentiates of the parental liquid by a process of gravity differentiation, in which sinking of ferromagnesian crystals, particularly augites, played an important part. The basalts of Dalmeny and Jedburgh types, which in Arthur's Seat have strong chemical resemblances, probably represent only slightly differentiated parental magma. Concentrations of soda-rich volatiles produced intense albitisation in the Markle basalts and the mugearites. It is considered that the mugearites of Arthur's Seat crystallised initially as Jedburgh or Dalmeny types, and were altered to their present state during a deuteric phase.An attempt has been made to reconstruct the eruptive history of the volcano. The magma source appears to have been replenished with parental magma at least twice during the period of activity. The first and last eruptions were of Dunsapie basalt; the albitised products appeared at a fairly late stage.