chap. in. of the Tr achy tic Series. 81 



this series have possessed an exceedingly imperfect 

 fluidity; and as it is evident that only matter thus 

 characterised would be subject to become fissured and 

 to be formed into zones of different tensions, in the 

 manner here supposed, we probably see the reason why 

 augitic lavas, which appear generally to have possessed 

 a high degree of fluidity, are not, 1 like the feldspathic 

 lavas, divided into laminas of different composition and 

 texture. Moreover, in the augitic series, there never 

 appears to be any tendency to concretionary action, 

 which we have seen plays an important part in the 

 lamination of rocks of the trachytic series, or at least in 

 rendering that structure apparent. 



Whatever may be thought of the explanation here 

 advanced of the laminated structure of the rocks of the 

 trachytic series, I venture to call the attention of geolo- 

 gists to the simple fact, that in a body of rock at 

 Ascension, undoubtedly of volcanic origin, layers often 

 of extreme tenuity, quite straight, and parallel to each 

 other, have been produced ; — some composed of distinct 

 crystals of quartz and diopside, mingled with amorphous 

 augitic specks and granular feldspar, — others entirely 

 composed of these black augitic specks, with granules 

 of oxide of iron, — and lastly, others formed of crystal- 

 line feldspar, in a more or less perfect state of purity, 

 together with numerous crystals of feldspar, placed 

 lengthways. At this island, there is reason to believe, 

 and in some analogous cases, it is certainly known, that 

 the laminas have originally been formed with their pre- 



1 Basaltic lavas, and many other rocks, are not unfrequently 

 divided into thick laminas or plates, of the same composition, which 

 are either straight or curved ; these being crossed by vertical lines 

 of fissure, sometimes become united into columns. This structure 

 seems related, in its origin, to that by which many rocks, both 

 igneous and sedimentary, become traversed by parallel systems of 

 fissures. 



