Note 473 — Chapter XXVII · GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUYS OF SCOTLAND

The infusibility of the felspar was well shown in some experiments on the rocks of the neighbourhood of Edinburgh, made at my request by Dr. R. S. Marsden, who subjected some of these rocks to fusion at the laboratory of the University of Edinburgh. Microscopic sections were prepared of the products obtained. The basalt of Lion's Haunch is peculiarly instructive. Its large labradorite crystals have resisted the intense white heat which, continued for four hours, has reduced the rest of the minerals to a perfect glass. We can thus well understand how large definite crystals of felspar should have survived or appeared in dykes and veins while the rock was still thoroughly liquid. The glass obtained from the Lion's Haunch rock is of a honey-yellow, and contains translucent tufted microlites. The iron forms beautiful dendritic films in the cracks. Altogether, the glass presents a strong resemblance to the palagonitic substance so abundant among the lapilli in the tuffs of the vents.