394 PKOF. J. W. JUDD ON THE TERTIARY AND 



increase or decrease of felspar in different parts of tlie rock, that I 

 can scarcely doubt of there being constant and insensible passages 

 in the rock-mass from the felspathic dolerite to the non-felspathic 

 peridotite. 



The structure of the dolerites is exceedingly interesting ; nowhere 

 in the British Islands am I acquainted with more beautiful illus- 

 trations of the ophitic structure ; the very similar dolerite which 

 forms the great intrusive sheet at Portrush in Ireland comes 

 nearest to it. The fractured surfaces of the rock exhibit the broad 

 faces of black crystals of augite, occasionally interrupted by the 

 enclosed felspar crystals. Under the microscope the augite, which 

 by transmitted light is of a rich brown tint, is seen to form great 

 crystals, the continuity of which is indicated by the persistency in 

 direction of the cleavage- cracks, and by their uniform tint when 

 viewed by polarized light. Within these broad crystals of augite 

 are enclosed innumerable rectangular crystals of plagioclase felspar 

 and rounded grains of olivine. 



In the accompanying peridotites the felspar almost completely 

 disappears. In some varieties, which may be classed as picrites, 

 we find a number of broad crystals of augite which enclose nume- 

 rous grains of olivine (see Plate XIII. fig. 4). In other cases the 

 grains of olivine become so numerous as to make up the mass of the 

 rock, and augite appears only occasionally in their interspaces. 

 The latter variety may be classed with the dunites. 



Under the microscope, the rich brown augite of the Shiant-Isles 

 rock exhibits gas- and liquid-cavities along its planes of fracture 

 and strain ; but these are seldom filled with solid material, and the 

 tabular inclusions producing Schillerization are, so far as my expe- 

 rience goes, never present in them. The augite of these rocks is 

 seen in some cases to pass into paramorphic hornblende. 



The olivine of these rocks is a very interesting mineral. In the 

 thinnest sections it exhibits a distinctly yeUow colour by trans- 

 mitted light. This colour is nearly as intense as in the fayalite of 

 the eulysite of Tunaberg, for an opportunity of studying which 

 I am indebted to Mr. Thomas Davies, of the British Museum. 

 This colour of the olivine is so marked and persistent that I can 

 scarcely doubt of our having a highly ferriferous olivine associated 

 with the dark ferriferous augite of the rock. As a rule, the 

 minerals of the Shiant-Isles rocks are remarkably fresh and un- 

 weathered, but the olivine in some of the specimens, which were 

 for the most part collected from fallen blocks washed by the sea, 

 exhibits a very partial serpentinization. Enclosures with solid 

 matter of black colour, and others containing liquids and gases 

 occur along the planes of fracture ; but the black stellate enclosures 

 are rare. Occasionally, however, black or dark-brown enclosures 

 are seen encroaching from the fracture -planes along planes parallel 

 to the optic axis of the olivine grain, and so crowded together as to 

 render the crystal black and opaque. 



In addition to the non-felspathic rocks, which we have classed as 



