MINERALOGY OF ST ANDREW'S. 153 



of St Andrew's by the name of the Rock and 

 Spindle, and from which it is distant about a mile 

 and a half. 



This rock is about forty feet in height. To- 

 wards the base, there is a spherical concretion of 

 basalt, in the form of five or six-sided lengthen- 

 ed pyramids meeting at the apex, giving to the 

 mass a stellate appearance. The mass is likewise 

 divided into concentric layers. The basalt con- 

 tains crystals of augite, v/ith olivine, and glassy- fel- 

 spar. This concretion of basalt is surrounded with 

 the tuff, into which it gradually passes ; and 

 must have been completely enveloped by it, pre- 

 vious to its partial wasting away by the action of 

 the sea and atmosphere. It may be mentioned in 

 this place, that the regular basaltic columns at the 

 Ely, are a portion of a spherical concretion con- 

 tained in trap -tuff. Here the concretion is only 

 about ten feet in diameter : at the Ely it is seve- 

 ral hundred feet. 



In other parts of the tuff, small masses of amyg- 

 daloid and basalt occur^ leading directly to the 

 conclusion, that the bed is partly a mechanical, 

 and partly a chemical deposit, since these rocks 

 imperceptibly pass into one another. 



If the regular forms of basalt induced Dr Hut- 

 ton to conclude, that they furnished proofs of the 

 action of a central heat, he would have found 

 considerable difficulty in applying his heat to these 



