40 DR. J. "W. EVANS ON A MONCHIQriTE FROM [Feb. I9OI, 



More recently, Mr. G. T. Prior [21] has noted the oecurrerjce of 

 rocks of the monchiquite-type on Fernando Noronha, associated 

 with the well-known alkali-magma rocks of that island. They are 

 very similar to the Bohemian rocks. The colourless groiindmass 

 is isotropic, except in one case when it feebly depolarizes. He 

 defers his decision as to its real nature. 



In an interesting paper published in the present year, Mr. John 

 S. Flett has described dykes from the Orkneys, which are very 

 similar to the Brazilian monchiquites [27]. He believes that the 

 colourless matrix, which in this case has a slight action upon 

 polarized light, is a glass which readily passes into analcime and 

 other zeolites (op. cit. p. 890). 



The question of the true nature of the monchiquite-groundmass 

 has also been discussed by Prof. Loewinson-Lessing [26] pp. 291-94. 

 who suggests that, if it be analcime, it may be the result of the 

 hydration of an original glass. 



II. Analcime as a Hock-eoeming Miner a.l. 



Attention was first drawn to the importance of analcime as a 

 rock-constituent by Tschermak, in his paper on the teschenites of 

 Moravia, where he describes it as intergrown with felspar in a 

 granular mixture, and sometimes amounting to as much as 27 per 

 cent, of the rock [2 & 3].^ liohrbach described it in the same rock 

 as without definite outlines, and filling the interspaces between other 

 minerals [8].^ 



Opinions have differed as to the origin of the analcime of the 

 teschenites. H. Mohl believed that analcime and natrolite were 

 formed by the decomposition of a glassy matrix [6], Rohrbach and 

 Zirkel consider that analcime is an alteration-prodtict of plagioclase 

 [8, 14], while Rosenbusch would derive it from nepheline [12] 

 p. 332 & [20] p. 279.^ I cannot find a suggestion that it was in any 

 case an original constituent. In some instances it is undoubtedly 

 secondary, but it seems quite possible that when it is intergrown 

 with felspar or forms the groundmass it may be original. If this, 

 be so, the rock may be regarded as an altered variety of the 

 monchiquites, with which it agrees in chemical composition^ except 

 that it contains a little more water. 



In 1890 Lindgren [9] described an ' analcite-basalt ' from the 

 Highwood Mountains (Montana), containing augite, analcime, 

 olivine, and magnetite-phenocrysts in a fine-grained matrix of augite, 

 analcime, and magnetite. The analcime has definite crystalline 

 boundaries, and occasionally shows double refraction. He believed 

 it to be original, because the other minerals (including olivine) are 

 in so fre&h a condition. In 1897 Mr. Whitman Cross reported 



1 It was first reported from these rocks by Glocker [1] in 1852. 

 ^ A similar rock with interstitial analcime has also been described frani 

 California [!()] p. 284 & [17] p. 27. See also [31] p. 191. 



3 See also [11] pp. 66, 78-79, [10] pp. 284-89, & [17] pp. 27-29. 



