ANALCITE GROUP OF IGNEOUS ROCKS 685 



iiK)nchi(|uitcs of Roscnbusch and the analcitc basalts of Lind- 

 gren are the same thing, the only difference being that the High- 

 wood Mountain types are lacking in the amj^hibole found in the 

 Brazilian ones. 



In the Highwood Mountain t3'pes, as described by Lindgren, 

 the analcite phenocrysts are sharply idiomorphic, which must 

 have been one factor in preventing Lindgren from falling into 

 the error concerning their nature which so many petrographers 

 have committed. Moreover from this fact it would appear that 

 the Highwood Mountain types are the best crystallized and most 

 individualized type of monchiquites which have yet been des- 

 cribed. Rosenbusch indeed classifies them with this group in 

 the last edition of his Massige Gesteine.^ 



It now seems probable that analcite as a rock component is 

 not limited strictly to the monchiquite group. The base des- 

 cribed in basaltic rocks by Bucking^ i^Basis zweiter Art) as a color- 

 less glass containing water, or which is stated to have the 

 general composition of nephelite, that is, consisting of silica, 

 alumina and soda and which gelatinizes readily with acids, is 

 more than probably analcite, and it is quite possible that all of 

 the colorless glasses which have been described as gelatinizing 

 readily with acids have this composition. It seems very unlikely 

 that a glass consisting of silica, alumina and soda would be 

 readily attacked by acids and gelatinize ; the basic glasses rich 

 in lime, iron and magnesia and approaching a slag in composi- 

 tion, that is an approximation to the formula RgSiO^, are at 

 times readilv dissolved by acids, but it is strongly to be ques- 

 tioned if a soda -alumina glass would be. The deteimination of 

 a colorless isotropic substance containing silica, water, soda and 

 alumina and which gelatinizes with dilute acids in a rock is as 

 safe a determination of analcite as that of the majority of 

 minerals determined in eruptive rocks. 



In the discussion of the primary or secondary nature of the 



''Ihird edition, 1895, p. 542-543. 



'Basaltische Gesteine, etc., Jahrb. k. k. preuss. geolog. Laudesanst. 18S0 and 

 1881. 



