498 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.59. 



occur here mostly v (103) with or without £ (308) and k: (3.0.10), which may replace 

 v (103). These three forms are always lacking in type 2 crystals, which often show 

 the typical rounded form ^ (201) and the negative n (101) and 2 (302). Another 

 important feature is the etching on these forms. On type 1 all the negative domes 

 are dull but give fairly good though faint signals on the goniometer, while on type 2 

 crystals v (103) and x (102) are completely lacking in reflection. The form v (111) 

 is a characteristic form of type 1, while on crystals of type 2 it is replaced by P (332) 

 as brilliant faces. Finally, only on type 2 crystals occur q (312) and T (314) with the 

 subordinate forms controlled by this interesting group, which includes t) (811) and 

 4> (546). The forms which are characteristic of the two types are as follows: 

 Type 1. Type 2. 



* (201) 

 £:(3.0.10) y'(601) 

 v (103) j (502) 



§ (308) 2 (302) 



n (loi) 



v (111) P (332) 



q (312) 

 »• (313) 

 r (314) 

 N (322) 

 X (534) 

 7(412) 

 1) (811) 



* (546) 



On crystals of both types individual forms may be lacking, but a mingling of forms 

 characteristic of the two types never occurs. 



According to Ungemach's definitions all of the crystals described 

 by Whitlock and by Kraus and Cook fall in type 1, Of those studied 

 by Gorgey and Goldschmidt only two (Nos. 5 and 8) are of type 1, 

 all others being of type 2. Of the eight crystals studied by Ungemach 

 himself Nos. 1 and 2 belong to type 1 and the other six belong to 

 type 2. So rigidly does he regard his definitions that he suggests 

 that Gorgey and Goldschmidt may be in error when they give the 

 form P (332), characteristic of type 2, as occurring on their crystal 

 No. 8, which otherwise agrees with type 1. 



In minerals which are liable to great variation in habit, such as 

 datolite, the development is commonly susceptible to variation with 

 variation in the conditions attending deposition. Variation of com- 

 position, temperature, and pressure of solutions depositing the min- 

 eral are ordinarily shown by differences in the habit of the crystals. 

 Where a single locality produces crystals of two or more distinct 

 habits the difference can usually be found to be due to the fact that 

 the crystals of different types belong to different generations. At the 

 Westfield quarries, as previously mentioned, the datolite seems to 

 have been deposited from a single set of solutions and the mineral in 

 all veins and cavities over a large area seems to be practically con- 

 temporaneous in time of formation or deposition. Only one genera- 

 tion of datolite is represented, and no other mineral is exactly con- 



