460 Alexander Scott — The Crawfordjohn Essexite. 



by Sears 1 to an olivine-free soda-gabbro from Essex Co., Mass. 

 Rosenbusch 2 expanded tbe term to include olivine-bearing types of 

 tbe same rock, and also applied it to the Norwegian rocks which 

 Brogger 3 had termed ' olivine-gabbro-diabase '. Owing to the fact 

 that the amount of nephelite may vary widely, while olivine and 

 hornblende may be absent or may be prominent constituents, the 

 term essexite has been very loosely used, and rocks ranging from 

 gabbro to theralite have been included in this class. The name 

 should be restricted to those gabbroid rocks in which the felspar is 

 strongly sodic and the nephelite small in amount. "Where nephelite 

 (or analcite) is a prominent constituent, the rocks should be classed 

 as theralites, while tbe name essexite-tberalite could be used for 

 intermediate types. Where olivine or hornblende is present in 

 notable amount, the terms ' olivine-essexite ' and ' hornblende-essexite ' 

 could be used. Thus some of the specimens from Craighead could be 

 included under the former name, while the latter could be applied to 

 the dominant type in the Monteregian hills. 4 



That the Craighead rock is related chemically to the theralites may 

 be seen by a comparison of columns 1 and 5, table ii, while the 

 mineralogical connexion is evident from a consideration of columns 

 1 and 4, table i. This type, therefore, might be fitly termed an 

 essexite-theralite, and the rock generally may be said to vary from 

 a normal essexite to an olivine-essexite and essexite-theralite. Some 

 of the olivine-rich varieties have obvious affinities with the less femic 

 types of kylite (cf. column 5, table i), and it is apparent that by an 

 increase in the amount of olivine a continuous sequence may be 

 traced from essexite through olivine-essexite to kylite and finally 

 kylite-picrite. 



The Marginal Rocks. 



Although several types of aphanitic rock can be recognized under 

 the microscope, these generally exhibit a monchiquitic habit. One 

 type, which occurs in the large quarry and may be distinguished in 

 the hand-specimen by the ' spotted ' appearance of the weathered 

 surface, is rather less aphanitic than the others. In thin section it 

 is seen to consist of abundant microphenocrysts of olivine and augite 

 with scarcer felspar in a fine-grained matrix. The augite, though 

 a titaniferous variety, is somewhat less pleochroic than the mineral 

 of the essexites, while the olivine is usually replaced by an aggregate 

 of fibrous serpentine. The ferromagnesian minerals are often 

 poecilitically enclosed in the labradorite, a structure strikingly in 

 contrast to that of the essexites, where augite is the enclosing 

 mineral. The matrix consists of small granules of augite and 

 magnetite in a dusty base of analcite and nephelite with locally 

 a number of small felspar laths. Mineralogically the rock might be 

 termed a nephelite or analcite basalt, but on account of its association 



1 Bull. Essex Inst., xxiii, p. 146, 1891. 



2 Mikroskopische Physiographic 4th ed., 1908, II, i, p. 404. 



3 Loc. cit. 



4 F. D. Adams, loc. cit. ; also Journ. Geol., xi, pp. 239-82, 1903 ; J. A. 

 Dresser, Geol. Surv. Canada, Mem. No. 7, pp. 14-18, 1910 ; J. J. O'Neill, 

 ibid., Mem. No. 43, pp. 28-84, 1914, etc. 



