178 Reviews — Dr. Washington — Igneous Rocks, Arkansas. 



themselves aifected in some measure by the central uplift, we may 

 perhaps plausibly picture the whole as consisting of two thin 

 laccolites bent into a gentle dome and exposed by erosion which, in 

 the centre of the area, has almost penetrated to the underlying 

 sedimentaries. In this view the central exposure of rock would 

 represent, not the centre, but the base of the lower laccolite ; and 

 the successive rings would represent, not inner and outer zones, but 

 lower and higher strata. 



Washington's petrographical investigation is of great value as 

 defining with precision the several rock-types recognized by 

 Williams. The outermost member of the complex is a foyaite, 

 consisting of orthoclase (51*8 per cent.), nepheline (20-3), cancrinite, 

 pyroxenes, etc. Next comes the rock for which the term covite is 

 adopted : it has points of resemblance with the shonkinites of 

 Montana, and may indeed be regarded as a nepheline-shonkinite. 

 Here felspars (orthoclase and albite) still make up more than half 

 of the mass ; nepheline is reduced to 9 per cent., and the principal 

 ferro-magnesian element is hornblende. In the third place comes 

 the very interesting rock in which Williams described the now 

 well-known pseudomorphs after leucite. This mineral is calculated 

 to have made up 36'9 per cent, of the rock, nepheline being 25'5, 

 orthoclase only 3'9, and the rest melanite and pyroxenes. This 

 rock, which Washington names arJcite, is, as Mr. Teall ^ has pointed 

 out, almost identical with the borolanite of Sutherland, although the 

 Arkansas type must have been richer in nepheline. 



The three rocks enumerated, which make up the outer ring, are 

 leucocratic (the white minerals predominating). The types met 

 with in the inner part of the Cove are melanocratic (with pre- 

 ponderant dark minerals), and here felspar almost wholly disappears 

 in favour of nepheline. The ijolite contains 38-7 per cent, of that 

 mineral, a smaller proportion than in the typical ijolite of Finland ; 

 of the rest, pyroxenes make 42*8 per cent, and melanite 15"3. Another 

 distinct type is recognized under the name biotite-ij'oUte. This 

 carries nepheline 24*1 per cent, and orthoclase 4'8, the rest being 

 diopside, melanite, and other dark minerals. Finally, there is 

 a jaciipirangite identical with the type-rock from Brazil ; this has 

 only 4 per cent, of nepheline, the remainder being chiefly pyroxene. 

 This rock is not found (may possibly be concealed) in the centre 

 of the area, where it should be expected, but is described from an 

 outlying locality. The centre, however, is occupied by a mass 

 essentially of magnetite, which produces great disturbance of the 

 compass in its neighbourhood. The mineralogical, no less than 

 the chemical, composition of the rocks indicates a very close con- 

 sanguinity for the whole assemblage ; but one may remark certain 

 differences, especially in the ferro-magnesian silicates of the several 

 types, which militate to some extent against the hypothesis of their 

 origin by diflfentiation in situ. 



In his general discussion of the mutual relationships of all these 

 associated rocks the author is necessarily upon less firm ground ; 

 ' Geol. Mag., 1900, p. 389. 



