290 GENERAL COMPARISON OF 



to the whole series, will be found comprised under the 

 following division, and they are placed as nearly as 

 possible with some regard to their relative importance, 

 or to the spaces which they respectively occupy. 



The first of these is a schist thickly and imperfectly 

 fissile ; either consisting of schistose chlorite and felspar 

 alone, or of these minerals with hornblende or with 

 actinolite superadded. This rock shows no external 

 marks of its fissile tendency, like micaceous schist or 

 ordinary chlorite schist; that property being detected 

 only by the hammer, to which it is at the same time 

 extremely refractory. Where exposed to the air, and 

 more particularly where it is washed by the sea, it 

 assumes rounded forms, with polished surfaces in every 

 part; indicating an uniform massive structure like that 

 of greenstone. In this case it is equally undistinguishable 

 in colour from that rock ; the chloritic ingredient becoming 

 black, and the felspar producing the same white specks that 

 are visible in the rocks of this species. By the increase 

 of the actinolite or of the hornblende, this stratum 

 becomes so compact as scarcely to be distinguished from 

 a hornblende or an actinolite schist ; into which it seems 

 ultimately to pass, being then undistinguishable from 

 those rocks as they accompany gneiss. A remarkable 

 circumstance is occasionally seen in these beds where 

 they undergo the transitions in question, as well as in 

 the ordinary hornblende schist that forms part of the 

 series. They often present a prismatic fracture at right 

 angles to the beds, the prisms being as regular and 

 decided as in the trap rocks. When this bed therefore 

 is uppermost, and attains the thickness of thirty or 

 forty feet, while the inferior are thin and present the 

 ordinary parallel stratification, it hence frequently as- 

 sumes the aspect of a superincumbent mass of trap so 

 strongly, that it requires great attention and minute 

 examination to make the distinction. The deception is 

 much aided by the composition, which is often similar 



