Cook and Kraus — Datolite from Great Notch. 643 



The crystals uniformly show a very pronounced tabular 

 development parallel to the hemi-orthodome a? {102} (see fig. 1). 

 Among the various types already described, the one under dis- 

 cussion approaches most closely to that described by Dana* 

 from Bergen Hill, N. J., which he characterizes as rhombo- 

 hedral and states is * a rare and quite unique form.' The 

 tabularity of the Great Notch crystals is however their most 

 striking characteristic. The pronounced similarity to crystals 



Fig. 1. 



from Guanajuato, Mexico, described by Farringtonf is also to 

 be noted. 



In all the following twenty-two forms were observed : 

 Pinacoids — a {100}, 6{010}. 

 Prisms — A {210 }, mjllO}, o{120}. 

 Clinodomes — m x \0\ I [, #{012}. 

 Orthodomes— x\ 102}, <£{101 }, j{502|. 



Pyramids — rc{ 111 },8{221 },S| 212 }, o \ 21 3 \,v{ 111 },c{l 12 \,X{ 113} 

 I^^l^ll Q ( 122£ a , 124 l - T" ( 134'- l\Q12\ 



- LJ \ L " L h H( J '-'- J )5 ^ ( iiJ *)J "'■if 1 - *)? I ( -'-!•-') • 



Of the above forms the orthodome a? { 102 ^ is always the pre- 

 dominating one. It is always heavily coated, and when the 

 coating has been removed, as indicated later, shows the effect 

 of natural etching so that a series of images is obtained, mak- 

 ing very accurate readings impossible. The prism m\110\ 

 occurs as a rhomboidal face with a coating making definite 

 readings difficult. The prisms A 5210} and 0J12OJ occur as 

 thin edges. Of the pyramids, n {111} and e {112} are always 

 comparatively large, e \ 112 } ranking next in size to x \ 102 j- , and 



♦Edward S. Dana, this Journal (3), vol. iv. pp. 16-22, 1872. 

 f O. C. Farrington. this Journal (4), vol. v, pp. 285-2S8, 1898. 



