TRIASSIC ERUPTIVE ROCKS. 409 



dicabase from the gneiss, its composition of pp-oxene and the trichnic feld- 

 spars labradorite and anortliite, and the rare presence of oUvine. By 

 comparison of the altered with the unaltered ^•arieties, it was seen that this 

 alteration has not been attended by further oxidation of the iron, and 

 therefore could not have been accomplished by any surface action, since 

 the oxidation of protoxide of iron is one of the chief causes of surface 

 alteration, while in this case one mineral containin<);- protoxide has been 

 changed into another protoxide mineral. It would therefore seem certain 

 that the alteration took place at the time of ejection, as had been urged by 

 Professor Dana.' 



Later, Mr. Hawes" made a separation (by Thoulet's solution) and 

 analysis of the feldspars in diabase from New Jersey, determining them 

 to be labradorite and andesite. He further calculates, on the basis of 

 analyses in his preceding paj^er, the mineral composition of the "West 

 Rock dike" near New Haven, finding it to contain the feldspars anorthite, 

 albite, and orthoclase, with augite, titanic iron, magnetite, and apatite. 



Professor Dana,' in a very jjungent critique of this paper, objects that 

 the anorthite came from a later transverse (east-west) dike in the West 

 Rock dike, and so can not be combined with the gross analysis of the latter, 

 as was done by Mr. Hawes. 



In 1882 the author published a paper, mainly mineralogical, on the 

 Deerfield dike,* in which the contact metamorphism on the sandstone below 

 and the amygdaloidal character of the trap sheet in its upper portion and 

 the unaltered condition of the sandstone above, which is molded into all 

 the interstices of the trap, are adduced in su])port of the view that this 

 trap body is a contemporaneous sheet and not a true dike. 



The presence of a beautiful fault at the mouth of Fall River was noted. 

 The proofs of its existence would seem to have been given with too much 

 brevity, as they failed to convince the author of the paper to be mentioned 

 next, and they will be given more fully in the sequel. 



In the following year apj)eared a very valuable article, by Prof W. M. 

 Davis,^ on the " Triassic trap rocks of the eastern United States," spe- 



' Am. Jour. Sci., 3d series, Vol. VI, p. 104. 



- Proc. U. S. Nat. Mus., 18«1, p. 129. 



' Am. .Jour. Sci., 3d series, Vol. XXII, p. 230. 



K\m. Jour. Sci., 3 series. Vol. XXIV, 1882, p. 195. 



sBull. C'omp. Zool. Harvard Coll., Vol. VII, p. 251. 



