O. 0. Farrington — Datolilefrom Guanajuato. 287 



hedral aspect and from those of both authors in the predomi- 

 nance of the clinodome t, 013. Through the kindness of Prof. 

 Dana, I have had an opportunity to examine some undescribed 

 crystals from Bergen Hill which resemble the Guanajuato forms 

 more closely. In these the clinodomes^ and g are still lacking, 

 their place being supplied by the pyramid G, but the habit, 

 mode of attachment and relative development of the faces is 

 such as to make the crystals in other respects very similar to 

 the Guanajuato forms. They differ from the Bergen Hill type 

 already referred to, chiefly in the subordinacy of e, 112. They 

 may therefore be considered as a simple modification of this 

 type. 



In order to illustrate the different types observed on the 

 Guanajuato datolites, four crystals, which are represented in the 

 accompanying figures, were chosen. Crystal No. 1 is shown in 

 completed form in fig. 2, fig. 1 showing the left half of the same 

 crystal. This crystal illustrates what is perhaps the commonest 

 type, the crystal being thin by reason of the shortness of the 

 clinodiagonal axis, and rather acutely terminated on account of 

 the predominance of the clinodome t. From the series of posi- 

 tive hemi-pyramids it will be noticed by the figure that the 

 pyramid X, 113, is wanting and this was usually true, the face 

 being found on only one crystal. 



Measurements in the zones of prisms, pyramids and domes 

 showed in all cases close approximation of measured to calcu- 

 lated values. A zone well brought out on this crystal is that 

 of x over n to x. Measured and calculated angular values in 

 this zone compared as follows : 





Measured. 



Calculated. 



]02 /N lll 



34° 20' 



34° 21' 



111^120 



29 42 



29 50 



120 xs Oil 



52 1 



51 53 



011/U02 



63 5V 



63 56 



Fig. 3 illustrates the second crystal chosen. Here the clino- 

 domes have a more nearly equal development than in the previous 

 forms and the crystal has, therefore, a more blunt termination. 

 The clinopinacoid is present, but only two of the series of posi- 

 tive hemi-pyramids occur, these being, as may be noted on the 

 figure, v, 111, and e, 112. Measurement of the zone x over n 

 on this crystal gave the following results : 





Measured. 



Calculated. 



102/slll 



34°2L' 



34° 21' 



111^120 



29 51 



29 50 



120^011 



51 54 



51 53 



011 A l02 



63 49 



63 56 



