AUGITIC ROCKS. 43 



Berwick is composed of trap-tufa, the masses being formed 

 of previously existing stratified and unstratified rocks im- 

 bedded in a green wacke. At Dunbar a trap-tufa occurs, 

 differing from the former in the circumstance of its being 

 highly impregnated with oxide of iron, which imparts to 

 it a bright brick-red colour. In structure, if it were not 

 for its mineral characters, trap-tufa frequently could not 

 be distinguished from true Neptunian formations, inasmuch 

 as it is distinctly stratified, and also assumes a slaty arrange- 

 ment. In its mode of connexion with the stratified masses, 

 trap-tufa exhibits characters perfectly distinct from those 

 which attend either the greenstones or basalts. It never 

 forms veins crossing strata, and consequently can never, 

 like them, be found as masses alternating in the same way 

 with strata, for these alternations are in every instance only 

 veins, which, instead of crossing the strata at angles, have 

 been injected between them from a central point of eruption, 

 and from a column of more or less fluid igneous matter, 

 which, to find egress, must have broken through the strata 

 at an angle. 



The comparison of the modern volcanic tufas with those 

 which are associated with the various trap-rocks, affords 

 one of the many proofs that the origin of both is the same, 

 modifying circumstances producing those differences which 

 cause both classes of rocks to be naturally separated. In 

 the volcanic tufa, there is the same stratified structure as 

 occurs in that of the trap family, and the series of alterna- 

 tions of volcanic tufa with volcanic basalts and lavas is 

 analogous to that of the trap-tufas with basalt, porphy- 

 ry, and greenstone. The circumstance of the tufas having 

 a stratified arrangement, and never occurring in the form 

 of veins, intimates that the manner of their formation was 

 intimately connected with the action of water, — that their 



