76 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



aggregates. 



Fig- 28.— Apophyllite showing 

 commonest combination of 

 pyramid with small prism 



FACES. 



It frequently rests on prehnite, stilbite, and datolite, 

 and is clearly earlier than apophyllite and 

 calcite. 



When first exposed in the quarry the lau- 

 montite crystals are water clear and colorless. 

 Upon drying out, however, they become 

 opaque and white from partial dehydration 

 and become very friable, falling to pieces by 

 splitting along the perfect prismatic cleavage. 

 This dehydration tends to swell the crystals 

 so that they do not yield accurate goniomet- 

 ric measurements. They are all simple in 

 habit, the forms being the unit prism, the 

 average angle measured on which was = 43° 

 15' or mAm'" = 93° 30' as compared with 

 93° 44', the value given by Dana. The term- 

 ination is always by an oblique face with an- 

 gles <^=90° 00', p = 35° 20' which indicate 

 that the form is the negative dome e (TOl), 

 the calculated p for which is 35° 40'. The 

 habit and appearance of the crystals, which 

 are all vertically striated, are shown in fig- 

 ure 26. 



Whitish opaque material which had been 

 exposed to the air of the laboratory for sev- 

 eral months was analyzed in the Museum lab- 

 oratory yielding the following results: 



Analysis of laumontite from Goose Creek. 



