ON THE MONCHIQUITES OR ANALCITE GROUP 

 OF IGNEOUS ROCKS. 



As IS well known to all petrographers the name of monchi- 

 quite was first given by Rosenbusch' in 1890 to a series of dark 

 basic dikes occurring in connection with intrusions of eleolite- 

 syenite in Brazil. Similar rocks had been previously known 

 from South Portugal and hence they received their name 

 from the Serra de Monchique. 



The material from Brazil was investigated by Hunter and 

 Rosenbusch, and the rocks which have a basaltic habit were 

 found by them to consist of ferro-magnesian minerals in a glass 

 base. The ferro-magnesian minerals were found to be always 

 olivine and pyroxene, while with them were associated some- 

 times amphibole, sometimes biotite, and sometimes both, and 

 according to these variations the group was subdivided. 



Since then these rocks, of slightly varying types, have been 

 found, usually in connection with intrusions of alkali syenites, in 

 various parts of the world. In this country they have been 

 found in Arkansas and in numbers in the Lake Champlain dis- 

 trict, and our knowledge of them is chiefly due to the researches 

 of Kemp.^ 



In all cases the main characteristics of the types described 

 are that the rocks consist of ferro-magnesian minerals, chiefly 

 olivine and pyroxene, lying in what is called a colorless glass 

 base. 



The only case in which, so far as we know, this colorless 

 base has been investigated was in the original study by Rosen- 

 busch and Hunter, in which it was separated and analyzed and 



'Tscher. Min. Mitt., Vol. XI, 1890, p. 445. 



= Trap Dikes, Lake Champlain Region ; Bull. 107, U. S. G. S., 1893. Igneous 

 Rocks of Arkansas; Ann. Rep. 1890, Vol. II, p. 392 (with J. F. Williams). 



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