CRYSTALLINE ROCKS. 25 



an obtuse ellipse. They sometimes occur so abundantly and with so 

 little common direction of their axes as to give the rock the appearance 

 of a conglomei-ate. 



The chief physical character which distinguishes this rock is its 



tendency to form bosses and tors ; the former 

 Physical characters, 



exhibit a perfectly smooth and dome-like outline, 

 which at first one is apt to suppose must be due to glacial action ; closer 

 examination, however, proves it to be a structure penetrating far beneath 

 the surface, and, in fact, that the bosses are not solid masses of rock, but 

 consist of a series of concentric shells, the thickness of which varies from 

 eight inches up to two feet. Different bosses exhibit various stages of 

 the break-up of these shells ; some have a curved fragment clinging on 

 here and there, while others exhibit a perfectly smooth unbroken surface • 

 a talus at the base being the only remains left of the last shell. 



No theory yet put forward has satisfactorily accounted for the 



origin of these shells, or rather we should say 

 ■ Theories. 



of the spherical joints which have given rise to 



them. It has been suggested that heavy showers of rain falling on the 

 surface of the rock which had become heated by the sun have produced 

 a sudden contraction and consequent splitting off of the shells; this 

 explanation, however, is obviously insufficient to account for any but 

 the very thinnest layers; the non-conducting powers of stone renderino" 

 it quite impossible that the sun's heat could penetrate to a depth of 

 two feet. 



The true explanation will probably be found to be that this struc- 

 ture was formed as the rock cooled down from the 

 Probable explanation. , . , . , , , • , . 



high temperature to which it may have been raised 



during the process of metamorphism. The fact of a somewhat similar 

 structure being occasionally seen in igneous granites and lavas favors 

 the idea that it is due to internal heat, 



( 133 ) 



