280 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 12 



Only those distinctive features of the two olivines that are readily 

 seen in thin section are given here. Chemical evidence and more 

 detailed optical measurements are available, but await the final 

 paper in which the characters of the other minerals and variations 

 within the mass will be discussed. 



An analysis of one of the freshest specimens has kindly been made 

 by Dr. H. S. Washington and is given in table 1. 



TABLE 1. 

 Chemical Analysis of Monticellite Alnoite. 

 Isle Cadieux, Quebec. H. S. Washington analyst. 



Sum 100.44 



This specimen was chosen primarily for its freshness and, while not 

 an uncommon facies, is not altogether representative of the usual 

 type. It contains more melilite than biotite and somewhat more 

 chrysolite than monticellite, whereas these conditions are reversed 

 in the average rock of the mass. If the amounts of CO2 and H2O be 

 taken as criteria, this is the least altered alnoite that has yet been 

 analyzed. 



Monticellite as an igneous rock mineral. — Monticellite, the lime 

 magnesia olivine, has hitherto been supposed to occur only in contact 

 metamorphic rocks. That it can crystallize directly from melts of 

 the appropriate composition has been shown by Ferguson and Merwin.^ 

 That it has not been found in igneous rocks before is in part due to the 

 fact that natural magmas have usually not been of appropriate com- 

 position and it was not present, but it may also be due to the fact that 

 it has, at times, escaped detection even though present. I have 

 examined a series of slides made from specimens from the original 

 Alno locality. Only one of these is of alnoite (labeled fine-grained 

 alnoite Aldersnaset, Alno^) and in this I find that many of the chryso- 

 lite grains are surrounded by rims of monticellite, always in optical 



6Amer. Journ. Sci. (4) 48: 81-123. 1919. 



« The specimens are the property of Dr. Washington and were collected by Dr. Hog- 

 BOM. A duplicate of this collection is in the U. S. Geological Survey Petrographic Reference 

 Collection, where this rock is No. 1044. 



