Large sandstone xenolith within the phonolite of Traprain Law

Abstract

In the year 1928 two large xenoliths, one of shale and the other of sandstone, in the phonolite of Traprain Law, were discovered, as described by the author. ( Trans. Edin. Geol. Soc., vol. xii., pt. ii., p. 252). Subsequently both these xenoliths were completely removed by quarrying operations. On 14th May 1930, it was found that the workmen at Traprain Law Quarry had uncovered a very large xenolith of sandstone, which measured, as it appeared in the phonolite, 29 feet 6 inches long, and 6 feet 9 inches wide (Plate L., Fig. 1). Its stratification was well preserved, and was parallel to the slanting joints in the phonolite. The xenolith, with the exception of a small part at the upper end, was not in any way distorted. The sandstone at its junction with the igneous rock was firmly welded to it, but at no point of the long straight contact of the two rocks was there any sign of intrusive tongues of the phonolite penetrating the sandstone. Very good specimens were obtained. A microscopic examination of a section (Plate L., Fig. 2), which included a junction of the two rocks, revealed features closely corresponding to those seen in the sandstone xenolith of 1928. There was a large development of felspar (orthoclase), much recrystallised quartz, and a large amount of granular ferromagnesian mineral (diopside or augite). It was considered that a chemical analysis of the sandstone would prove interesting. Two samples were taken, one at the actual point of contact, and