On the Norian or " Upper Laurentian " Formation. 279 



other of the above-mentioned causes is exhibited more or 

 less distinctly by the rocks of all the anorthosite areas 

 that have been investigated ; but it is not confined to 

 these, since it has been observed in many gabbros and 

 basic plutonic rocks allied to them in districts widely 

 separated from one another. 



Dr. Geo. H. Williams, for example, says, in his treatise 

 on " The Gabbros and Associated Hornblende Rocks 

 occurring in the neighbourhood of Baltimore, Md.," 1 on p. 

 25 : " The most striking feature in the texture of the 

 unaltered Gabbro is the repeated and abrupt change in 

 the coarseness of the grain which is seen at some localities. 

 It was undoubtedly caused by some irregularity in the 

 cooling of the original magma from a molten state, for 

 which it is now difficult to find a satisfactory explanation. 

 The coarsest grained varieties of the Baltimore Gabbro 

 occur in the neighbourhood of Wetherville, and there 

 these sudden changes in texture are most apparent. 

 Irregular patches of the coarsest kinds lie imbedded in 

 those of the finest grained without any regard to order. 

 In other cases a more or less pronounced banded structure 

 is produced by an alternation of layers of different grains 

 or by such as have one constituent developed more 

 abundantly than the others. Such bands are not, how- 

 ever, parallel, but vary considerably in direction and show 

 a tendency to merge into one another as though they had 

 been produced by a motion in a liquid or plastic mass." 



Similar and very coarse-grained portions are also found 

 in the gabbro-diorite which is quarried at Kiihlengrund, 

 near Eberstadt, in Hessen, a rock which is otherwise quite 

 massive and of an even texture. Other occurrences might 

 easily be adduced. 



The most remarkable example which I have observed, 

 and especially notable for the reason that it shows the 

 transition from a perfectly normal massive rock through 



1 G. H. Williams, Bulletin 28, U.S. Geol. Survey. 



