Dec, 1908. Mineralogical Notes 135 



The habit of the simple crystals is uniformly tabular with respect 

 to c (100). They are usually also slightly lengthened in the direction 

 of the macro-axis. The planes are bright and sharply outlined and 

 give good signals with the reflecting goniometer. In length, in 

 the direction of the brachy-axis, the individuals vary from 1 to 2.5 

 centimeters. Their average thickness is about 5 millimeters. They 

 are rarely highly modified. They are usually made up chiefly of 

 three pinacoids and the unit prism, pyramid and brachydome. 

 Striations parallel with the edge m a usually characterize the prismatic 

 zone except for planes of m, a and b, which are smooth and bright. 

 Fig. 1, PI. XLVII shows the usual type. The development of the 

 prism is, however, not as a rule as well-defined as indicated in the 

 figure, the zone from a to m being often considerably rounded and 

 showing no well-marked planes. There may also occur a rounding 

 of this sort between m and b. Such rounding is, in fact, quite char- 

 acteristic. Some crystals are somewhat more highly modified than the 

 above. These show as a rule several pyramids and an increased num- 

 ber of prisms. Figure 2, Plate XLVII (Mus. No. M 7197) illustrates 

 such a crystal. This crystal was about 1 sq. cm, in area and 2 mm. 

 thick. 



Perhaps the most interesting type presented by these barites is 

 that already mentioned in which numbers of smaller crystals com- 

 bine to produce a crystal of different habit. The most common form 

 of these is illustrated in Fig. 3, PI. XLVII. Here small primary crys- 

 tals, chiefly made up of the basal plane and unit prism, have grown 

 together in parallel position to form a crystal of tabular habit which 

 shows essentially the planes c (001), a (100), and o (on). Here, there- 

 fore, the crystallizing force controlled the arrangement and situation of 

 the individual crystals as well as that of the molecules in the crystals 

 themselves. The crystal here illustrated is not doubly terminated in 

 the direction of the a axis, but in all other directions is fully developed. 

 The size of these large polysynthetic crystals is from 3 to 6 centimeters 

 in the direction of the a axis, and in the direction of the c axis about 

 one-fourth of this. Groups of diverging crystals which have no ap- 

 parent regularity, also occur among the specimens. 



The total forms observed on the Cartersville barites and some of 

 the measurements obtained are as follows: 



c (° QI ) X ( x 3o) / (n3) 



