Ah la i a aac a a a ta a NS a ek a a ee 
FE. S. Dana on the Datolite from Bergen Hill, N. J. 19 
The clinodome 4-2 is also occasionally wholly or in part re- 
placed by a convex +m-n plane (r), in the zone that includes 4, 
248n' In fig. 6 the 
plane (r) occurs, though by the projection its shape is not satis- 
factorily shown. Measurement gives for tA71=91° 30/ to 92° 
30’ near its intersection with —2-7; but for the larger part of the 
plane t,¢-2=97° —98°, and near its intersection with $ TALIS 
107° 20’-108°.. The last angle corresponds to the plane 8.3 fh 
whose calculated inclination on 7-7 is 107° 36’; while the main 
part of the plane has for » probably 4, ret ing 8-4, which in- 
clines toward 7-7 at the angle 97° 81’. re may be included 
in t the plane ¢ -6, for which the same are is 95° 13’. The 
plane 1-2 belongs to the same zone; but its angle with ¢? is 
103° 36’, and evidence of its presence ‘was not found, while that 
of 2-4 was sustained by the following observation. 
4-2 and 4, which plane therefore has m= 
Between 1 and —2-7 a convex plane (y fig. 15) occurs, 
giving 7-?~7=105°-107°; from the zone we have m= - 
—1 
This plane is in a zone between 1 and the position of 2-2, while 
3-4 is in that between $ and 4-2 The two zones here cross, 
and if 7 is the plane common to them, it is $-4. That it is so 
is rendered almost certain by its angle of Sie a on 7-7, and 
its similarity to t in having a convex su 
he convex planes & and t replacing the clinodomes 2-2 and 
4-1, sometimes occur together. In fig. 6, the intersections of O 
with the two planes t converge ackward, a necessary conse- 
quence of the oblique position of 1. 
The series of planes above 7-7, -4.i and -6-1 rand the adjoining 
In mee to the ae character of different planes, and 
their frequency of occurrence, I make a few additional remarks. 
The clinodiagonal hemi-octahedral planes and vertical prisms, 
almost without exception, are destitute of any polish, and often 
eg rough ; this is true also of —2-¢ ae: athe other oe 
—6-3 6-2 44,42, and 4 The nr mining planes, sche 
cecal exce tions, are well polished, ae ae e presence 0 
wavy lines on t the surface in most. cases prevents vey cocce 
measurements, O, when of sufficient size to be well observed, 
is uniformly striated in two different directions, parallel to its 
intersections with the tahedral planes of the m series. In the 
crystals from which fig. 8 was drawn, it consisted of an oscilla- 
