32 N. L. Bowen — Genetic Features of 



Such straining" or filtering of liquid from crystals I 

 have elsewhere suggested as a dominant factor in the 

 formation of those unusual types that occur only as com- 

 plementary or diaschistic dike rocks. 32 A change in the 

 nature of a dike along its strike is a feature that might 

 be expected on this supposition, and Shannon has recently 

 found such changes in lamprophyres of Idaho. 33 



For a discussion of certain reaction effects in analcitic 

 rocks the reader is referred to papers by Tyrrell 34 and 

 by Scott. 35 



Summary . 



Alnoitic rocks at Isle Cadieux near Montreal, Canada, 

 are found to consist principally of monticellite alnoite, 

 a newly recognized (not a new) rock type. Besides this 

 type there is a variety consisting almost exclusively of 

 melilite and biotite, which is apparently new. 



A study of the relations of the minerals indicates that 

 the rock originally consisted of augite and chrysolite and 

 was nearly completely consolidated as such. These min- 

 erals were then attacked, with lowering temperature, 

 probably by their own interstitial liquid as it changed in 

 composition, and were replaced by monticellite, melilite 

 and biotite with marialite, perovskite and titaniferous 

 magnetite as minor products of the reaction. The monti- 

 cellite is itself replaced by melilite and biotite and the 

 melilite-biotite rock is the end product of the replacement. 



The replacement was accomplished by an alkalic 

 liquid (magma) which formed monticellite from augite 

 by desilicating it and later gave rise to melilite and the 

 more definitely alkalic mineral biotite. 



The melilites are both positive and negative and are 

 sometimes conspicuously zoned, with a positive core, an 

 isotropic intermediate zone, and a negative rim. 



In an experimental part of the paper equilibrium in 

 mixtures of nephelite and diopside is determined. It is 

 found that from intermediate mixtures forsterite and 

 melilite are the first products to crystallize and the 



32 Proc. Nat. Acad. Scl, 6, 161, 1920. 



33 Proc. U. S. Nat. Museum, 57, 1920, p. 480. 



34 G. W. Tyrrell, Quart. Jour. Geol. Soc, 72, pt. 2, 84-131, 1917. 



35 Alex. Scott, Trans. Geol. Soc. Glasgow, 16, pt. 1, 34-45, 1915-16. 



