Vol. 57.] MOUNT GIKNAR, JFNAGAEH. 51 



Subsequently to the formation of the rock further changes appear 

 to have taken place. Some portions of the original analcime were 

 transformed into alkali-felspars — orthoclase, microperthite, and soda- 

 microcline — containing more silica, and nepheline containing less 

 silica than the original analcime. The portions where this change 

 has taken place may be described as hornblende-tinguaite. 



It appears, from the analyses of the analcimes that have been 

 formed under pressure from fusion, that they contain a much 

 larger percentage of potash than those resulting from the decom- 

 position of igneous rocks at moderate depths by underground 

 water : compare analyses l^os. 116, III, IV, & V, p. 47. It seems 

 probable that under high pressure analcime may be able to form with 

 a larger proportion of potash, and that when the pressure is released 

 the presence of the potash renders it a less stable compound.^ 



The decomposition of the analcime appears to be similar to that 

 of leucite in the ' pseudoleucites' of Brazil [28], Arkansas [11], 

 and Montana [29], except that in those cases the original mineral 

 contained no water.^ In the pseudoleucites the arrangement of the 

 felspar and nepheline is very regular, while here, where the whole 

 structure is on a smaller scale, the nepheline has chiefly collected in 

 the spherical spaces and the felspars in the interstices of the other 

 minerals. 



The normal decomposition of analcime ^ may be represented by 

 the following equation : — 



15 ( E'20,Al,O3,(SiO,),(H2O)2 1 =7 jR'oO,Al,0,,(Si02)6 1 +2 U'R'^OAkOM^iO^)^ i +3OH2O 

 < Analcime. ^ ' Alkali-felspar. ' ' Nepheline. ' 



Much more nepheline is, however, present than can be thus 

 accounted for. A good deal of it is probably original, and crystal- 

 lized out before the analcime. 



With regard to the inclusions, the apatites seem to have been 

 formed at a very early period, but the others can scarcely be older 

 than the nepheline and analcime, for their arrangement and orienta- 

 tion are related to the structure of those minerals. 



In the present rock, the existence of the symmetrical spherical 

 spaces which have been so often mentioned in the foregoing pages 

 is inconsistent with the contention which has been advanced in 

 similar cases,^ that the analcime is the product of the hydration of 

 a glassy matrix. Now that it has been shown that analcime is in 

 many cases a primary rock-constituent, the most obvious explan- 

 ation of the formation of these spheres is that they are the result 



^ The portion of the matrix which has broken up into felspar and nepheline 

 may represent analcime that contained a larger percentage of potash than that 

 which remains. 



^ This is on the assumption that it was leucite ; it may have been an 

 analcime with a high percentage of potash. The essential difference between 

 the minerals lies, not in the proportion of alkalies, but in the presence or 

 absence of two molecules of water. The percentage of soda in the Arkansas 

 specimens analysed by J. R. Williams was much higher than that in any 

 recorded analysis of leucite. 



^ The onlv difference in the case of leucite is the absence of the water, 



^ See [6] ;' also [20] p. 539, [24] p. 233, [26] p. 293, & [27] p. 890. 



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