OLD KED SANDSTONE VOLCANIC ROCKS OF SHETLAND, 383 



structure which it presents under the microscope. A section prepared from 

 the knobs of this rock, near the church, on the east side of the island, displays 

 this spherular arrangement with singular beauty. The spherulites do not 

 traverse the section in regular bands, but in wavy lines and in groups. No 

 nucleus is observable in the centre of the spherules, nor are they enclosed by a 

 periphery, as in the spherulitic glassy lavas of younger date. "When magnified 

 120 diameters, some of these spherular developments exhibit very clearly the 

 fine radial fibres diverging from a common centre, while others have lost all 

 traces of these fibres. In the latter case they are supplanted by an exceedingly 

 fine-grained ground mass, but the rude outlines of the original spherules are 

 still visible. The ground mass is stained by minute ferruginous particles, 

 resulting from the decomposition of iron pyrites. There are numerous small 

 nests or cavities in the ground mass, filled chiefly with quartz of secondary origin 

 and small crystals of orthoclase. 



Another section, taken from the cliff at the Horn of Papa, exhibits a 

 similar spherular structure, though of a less perfect type. The divergent fibres 

 are not so characteristically developed, and in the parts of the section where 

 they are absent, the ground mass is more granular. The nests filled with 

 secondary quartz do not form such a striking feature in this section ; but one 

 large crystal of clear quartz, with fluid inclusions, occurs in the mass. 



Attention has been previously directed to the microscopic characters of 

 this felsite by Dr Archibald Geikie, who has pointed out the spherular group- 

 ings of the constituents and the absence of any crystalline structure in the 

 rock. From the evidence now adduced, we are inclined to believe that the 

 felsite originally possessed a vitreous character, which has to a large extent 

 disappeared through devitrification. In all likelihood the great alteration 

 which the rock has experienced may have been partly coincident with, and 

 partly subsequent to, this molecular change. If the vitreous material had 

 reached the surface originally, it would have been ejected in the form of a 

 glassy lava. 



The dykes radiating from the intrusive sheets present microscopic characters 

 of a varied type. The coarse-grained aplites or binary granites are identical 

 with the Northmavine and Sandsting sheets. A typical example occurs at 

 Mavis Grind, in which the felspar is mainly orthoclase with some Carlsbad 

 twins. Some microcline is also present, but the felspars have undergone con- 

 siderable alteration. The quartz occurs in isolated crystals and in groups 

 enclosed in the felspars. A. number of sections of the quartz-felsites have been 

 prepared, displaying very uniform characters. A typical example from Aith 

 Ness, in Aithsting, shows a micro-crystalline ground mass of felsitic matter, with 

 small crystals of orthoclase porphyritically developed. Quartz occurs in small 

 grains and nests, and magnetite is also present in minute grains. Another sec- 



VOL. XXXII. PART II. 3 p 



