OVERLYING ROCKS. 475 



The next variety of structure to be noticed, 

 is the laminar. This appears to be sometimes 

 independent of the action of the atmosphere; at 

 others, it is obviously produced by that cause. 

 The laminae vary in dimensions, being sometimes 

 so thin as to resemble those of argillaceous schist, 

 and having thus given rise to the improper term 

 of porphyry slate. This structure occurs both in 

 masses and veins, and in all the varieties of these 

 rocks. It is also sometimes combined with the 

 columnar form, arid, in this case, the laminae 

 are sometimes parallel to the column, at others 

 transverse. In most of these rocks in which it is 

 found, namely, in the claystones, basalt, and the 

 porphyries, it is independent of the disposition 

 of the integrant parts ; but, in hypersthene rock, 

 it depends on the disposition of the crystals of 

 the characteristic mineral, and the structure thus 

 resembles that of gneiss, being foliated, strictly 

 speaking. 



A minute spheroidal structure, sometimes 

 also occurs in these rocks. It is very conspi- 

 cuous iu the rock of the Shiant Isles, but does 

 not appear common. The spherules are not 



