Fairbanks.] 



A nalcite Diabase. 



287 



Teall* describes a very similar rock consisting of prismatic 

 augite, feldspar much altered, analcite, natrolite, magnetite, brown 

 mica, and green decomposition products. He says: "Unless soda 

 has been introduced, the abundance of analcite would imply either 

 the existence of nepheline or of a soda-lime rather than a lime-soda 

 feldspar." 



The description given by Williamsf of the eleolite syenite rocks 

 of Arkansas is very interesting in connection with the study of this 

 rock. Several short extracts will be given to illustrate the struc- 

 tural affinities of those rocks with the Cuyamas eruptive. He says: 

 "Eleolite never occurs in this rock in idiomorphic crystals, but al- 

 ways in wedge-shaped or polyhedral masses which take their exterior 

 form from the minerals by which they are surrounded. * * * 

 It is a very common thing to find this mineral altered to analcite; in 

 some cases so complete has been this change that in many sections 

 no remnant of the original substance can be found. * * * The 

 analcite seems to have eaten into the feldspar wherever it could get 

 a chance, and many feldspars may be found whose centers have 

 become almost entirely changed into analcite, while others have 

 been eaten away on one side, the rest remaining intact." WilliamsJ 

 also describes a miarolitic structure in the Magnet Cove eleolite 

 syenite. The rock is much decomposed and the eleolite has nearly 

 disappeared. "It contains many cavities into which the feldspar crys- 

 tals extend and show by their perfect crystalline forms that these 

 spaces were originally empty or were filled with some late formed min- 

 eral which has since been decomposed." This description will apply 

 to both the Cuyamas and Oceanic dikes, both augite and feldspar 

 projecting into the analcite areas with perfect idiomorphic forms. 

 (Plate 1 5 , Fig. 6.) It is probable that these dykes were not originally 

 miarolitic, but that the spaces must have been filled by some late 

 formed mineral rich in alkali. It would hardly be reasonable to 

 suppose that a miarolitic structure existed so uniformly in all the 

 dikes described, both large and small. Carry a step farther the 

 decomposition of the eleolite of the Arkansas rocks, and we have a 



^British Petrography, p. 191. 



•("Arkansas Geological Survey, Igneous Rocks, pp- 66, 79. 

 %Ibid., p. 232. 



