3 ST. KILDA. GEOLOGY. 



the rude crystallizations of felspar and the smoky quartz 

 being also prevalent in the latter rock. 



The remaining, and by far the larger part of the 

 island, is composed of a dark trap rock. I can only 

 pretend to describe such varieties as are observed on 

 the surface of the land ; since it is impossible, either 

 from sea or land, to approach the cliffs so near as to 

 form any judgment of the different modifications they 

 may contain. They are all however obviously composed 

 of this substance ; and I may add that no columnar forms 

 were apparent, but that some portions seemed to indicate 

 that looseness of texture which belongs to the amyg- 

 daloidal varieties. The specimens visible on the surface 

 present generally the character of a greenstone, abounding 

 in hornblende and often passing into basalt. A dark 

 lead-grey compact felspar with little or no hornblende, 

 is also occasionally found. In many situations, the rock 

 contains large concretions of the latter mineral; which 

 are as usual much more distinct on the weathered surfaces 

 than in the internal fracture. They assume at times 

 a singularly serpentine or reticulated disposition, and 

 at others, appear like the heads of nails driven into 

 the surface. Augit also forms in some places an integrant 

 part of this rock, and it is perhaps sometimes this sub- 

 stance which I have here called hornblende ; but the diffi- 

 culty of distinguishing these minerals, when they are 

 found in an intimate state of mixture, is very great. 



The trap thus described is not disposed in terraces, nor 

 has its fracture on the great scale the vertical tendency 

 which that rock so commonly assumes. It is spiry and 

 irregular, and thus produces the craggy uneven surface 

 on which the sea fowls breed; the broken fragments 

 and soil accumulating in various places so as to form 

 small grassy slopes. 



As far as the relations of the trap and the syenite can 

 here be ascertained, by a distant view of these high 

 cliffs from the turbulent sea below, or from a perilous 



