182 ANNALS NEW YORE ACADEMY OP SCIENCES 



relations which they bear to each other and to other minerals. In such 

 cases as observations on sequence have been made, the order appears to 

 correspond in most instances to that observed in the Watchung series. 

 There are a number of instances recorded, however, of pseudomorphs of 

 minerals which, from observations of the Watchung series, should be of 

 early formation, but which have taken the form of zeolitic species. Preh- 

 nite and orthoclase appear especially likely to do this. This may pos- 

 sibly be due, as was suggested before, to a reversal of the normal sequence 

 by a change of conditions due to external agencies. On the other hand, 

 the data available are not sufficient to exclude the possibility that this 

 was a normal order imder the conditions of the case and that such factors 

 as the composition of the original rock may enter in determining the 

 sequence of deposition. 



In discussing in previous pages the application of the phase rule to 

 the problem and again in describing analcite, the inference was drawn 

 that analcite and natrolite should possess a transition point, at which sin- 

 gle point only they could coexist in equilibrium; and the observed rela- 

 tions of the two were found to confirm this conclusion. Two instances 

 taken from Hintze^' add further evidence : On the Kunetitzer Mountain, 

 Bohemia, analcite crystals are found 



up to 12 mm. in size in cavities of the basalt ; often porous and covered with 

 natrolite ; 



and again in the Tyrol 



at the Cipit-Bache at the north foot of the Schlern-Gebirg, milk-white to 

 flesh-red crystals, scarcely more than 1 cm. in size, covered with apophyllite, 

 more seldom with calcite and natrolite-needles ; many crystals porous and 

 altered to an aggregate of small needles (? natrolite). 



Heulandite and stilbite are believed to possess a similar transition 

 point. The well-known German mineralogist, Breithaupt, devoted special 

 attention to paragenesis of minerals. His work dates back sixty years or 

 more but as far as macroscopic observations go appears to be accurate 

 and reliable. In regard to heulandite and stilbite, he records the sequence 

 (a) heulandite, (b) stilbite, and remarks:^* "This paragenesis is fol- 

 lowed with perfect constancy in many localities of occurrence." It should 

 be noted that in Breithaupt's usage, stilbite was called "desmin" and 

 heulandite "stilbit." He gives numerous examples of the order observed 

 for various other zeolites and for quartz, prehnite, datolite and calcite 



"• Op. cit., p. 171". 



*" "Die Paragenesis der Mineralien "' Fraibere. o. 105, 1849. 



