52 



Recent Excavations at Stonehenge. 



All of these various features may be found in different specimens 

 of the sarsens which occur among the fragments found in the 

 excavations at Stonehenge. Of the relative paucity of the fragments 

 of sarsen found in the excavations, when compared with those of 

 the " Milestones " and other rocks, I shall have to speak further in 

 the sequel. 



II. Ophitic Diabase. — These rocks have been fully described by 

 Professor Maskelyne, and all the varieties which he mentions as 

 occurring in the " bluestone " monoliths of Stonehenge find their 

 parallels among the fragments collected in the recent excavations, 

 the rocks differ greatly in size of grain, some being coarse-grained 

 and porphyritic, others being comparatively fine-grained and almost 

 compact in structure. The ophitic character is in some cases much 

 more marked than in others. The rocks differ also greatly in the 

 amount of alteration they have undergone. In some cases the 

 felspar (labradorite ?), the augite, and the magnetite or titanoferrite 

 are all quite fresh and unchanged, so that the rock might be called 

 an "ophitic dolerite." More frequently the felspar has been 

 largely converted into epidote and zoisite (with secondary albite), 

 the augite into various chloritic minerals, and the titanoferrite 

 into leucoxene. 



III. Highly altered Basic Tuffs and Agglomerates. — These rocks 

 are generally of a fissile character, and some of them have been 

 variously referred to by different authors as " schistose rocks " and 

 "calcareous chloritic schists." They consist of chlorite and 

 leucoxene, with various colourless minerals (zoisite, albite, etc.), 

 the products of the alteration of minerals found in basic lavas. In 

 many cases the rock is evidently made up of more or less angular 

 fragments, and not unfrequently these fragments are vesicular, the 

 vesicles being filled with crystalline calcite. In consequence of 

 this, most of these rocks effervesce freely with acid. It is evident 

 that the rocks are of volcanic origin, and are ancient basic tuffs 

 and agglomerates. In some cases the amount of alteration which 

 they have undergone is extreme ; only traces of their original clastic 

 character can be detected, and the rocks have evidently become 

 more or less foliated in structure. The number of fragments of 



