KERRERA. GEOLOGY. 123 



of the simpler varieties occur, of a brown, grey, reddish, 

 or white colour, with different degrees of hardness and 

 much diversity of fracture ; offering specimens, to none of 

 which, in the present state of our nomenclature, it is pos- 

 sible to apply names that could be understood. It is per- 

 haps better to leave such substances without a name, than 

 to designate them by terms that have been already too 

 often used in a lax and ambiguous sense; since the ne- 

 gative confusion that may result from the want of appro- 

 priate appellations, is much less inconvenient than the 

 positive one which attends their misapplication ; inasmuch 

 as it admits of future amendment, and as it is much 

 easier to add to a nomenclature than to change its 

 signification. Zeolites, calcareous spar, and green earth, 

 occur in the amygdaloids, but the most remarkable 

 variety is found at the south-western part of the island. 

 This is amygdaloidal and of a somewhat earthy texture, 

 the base being harder than common clay stone. Toge- 

 ther with calcareous spar, it contains a large proportion 

 of brown mica ; some of which is perfectly crystallized 

 in hexagonal plates attaining a quarter of an inch in 

 diameter. This rock is brown, but weathers to a dark 

 red, and assumes irregularly columnar forms. 



If the connexions of these rocks with those on which 

 they lie are examined, it will be found that they are some- 

 times in contact with the clay slate, at others with the 

 graywacke, a proof of their posteriority to both, and of 

 their irregularity of position. They are also found branch- 

 ing into large veins, which again ramify into smaller; 

 penetrating the schist in various directions, and some- 

 times for a long space conformable to its disposition* 

 Where these parts only are visible, and where their 

 connexion with the superincumbent masses cannot be 

 traced, they have the appearance of beds alternating with 

 the schist; but this appearance, traced in one instance 

 to its true origin, justifies us in assigning the same to all 

 similar masses. 



