Alnoitic Rocks at Isle Cadieux, Que. 29 



Daly's assumption, for it is conceivable that we are 

 dealing with a condition of equilibrium whose position 

 may be varied under different conditions. 



The occurrence of monticellite in alnoite is another 

 example of the same sort of thing. It is a mineral which, 

 like vesuvianite and garnet, is found in metamorphosed 

 limestones, but it is commonly believed to be confined 

 to such a mode of occurrence. However, there is in the 

 collections of Dr. H. S. Washington a series of slides 

 from the original Alno locality, 22 only one of which is of 

 alnoite (labeled fine-grained alnoite, Aldersnaset, Alno) 

 and in this slide I find that many of the chrysolite grains 

 are surrounded by rims of monticellite, always in optical 

 continuity with the chrysolite. Monticellite embays 

 chrysolite and occasionally fingers through it. (See 

 Plate I(f ) .) The reaction relation is very plain. In fact, 

 while one cannot speak with confidence from the evidence 

 of a single slide, there 'is much in this alnoite of the type 

 locality suggestive of the same reactions (replacements) 

 that have been detailed here for the Quebec occurrence. 

 It seems not improbable that monticellite may occur in 

 other examples but has been overlooked. It would be 

 rather easy to do so in the presence of ordinary olivine 

 (chrysolite), particularly in cases where their manner 

 of occurrence was not so distinctive as in the present case. 

 Indeed, O'Neill describes "an unknown colorless min- 

 eral, ' ' occurring in a much more salic type of rock in this 

 same Monteregian province, whose properties as given 

 agree excellently with those of monticellite. It occurs 

 in a nephelite-sodalite syenite close to its contact only. 23 

 If it is monticellite its occurrence in the quickly chilled 

 contact variety of the rock and its absence elsewhere is as 

 one might expect, for in the more slowly cooled part its 

 place is taken by other minerals just as ordinary olivine 

 may occur in the contact facies of, say, a diabase which is 

 elsewhere free from olivine. 



On the whole, then, it seems not unlikely that mon- 

 ticellite may be of reasonably frequent occurrence in 

 alkalic rocks. Indeed this is the more probable since the 



22 The specimens were obtained from Dr. Hogbom. A duplicate of this 

 collection is in U. S. Geological Survey Petrographic Keference Collection, 

 where this rock is No. 1044. 



23 St. Hilaire and Eougemont Mountains, Quebec, Geol. Survey. Can., 

 Memoir 43, p. 42, 1914. 



