W. E. Ford — Remarkable Twins of Atacamite. 21 



fliiehige Yerwachsung," " Einzonige Yervvaclisuiig."* It is to 

 be expected, as attention is called to these twins of lower 

 symmetry, that more examples will be discovered and still 

 other groups formed. In fact the twins described in tins 

 paper do not seem to exactly tit into any of the groups in 

 Goldschmidt's scheme, resembling most nearly, however, his 

 group of "hetero- twins." A hetero-twin of orthoelase recently 

 described by Paul and Goldschmidtf is in some respects sim- 

 ilar to these twins of atacamite. The law governing the ortho- 

 elase twin is stated as follows : the face c (001) of ISTo. II is 

 parallel with h (010) of No. I ; the zone c (OOl)-m (110) of 

 No. II corresponds with zone h (OlO)-c (001) of No. I, and face 

 I (010) of No. II falls in the zone c (001)-?/ (201) of No. 1. 

 Here we have in the two individuals a parallelism of two 

 nnlike faces, two unlike zones falling together and a prom- 

 inent face of one individual falling in a prominent zone of the 

 other. In the case of the atacamite twins we have, on the 

 other hand, two similar faces in parallel position and a promi- 

 nent face on No. II lying in a prominent zone of No. I, and 

 in general the faces of No. II lying near to faces or zones of 

 No. I. 



Fig. 5. 



Fig. 6. 



Fig. 7. 



The twin crystals are of two types. The first, which is illus- 

 trated in figures 5 to 7, is of an interpenetration type where 

 one individual grows through another or projects from it. 

 Usually in this type one individual is much larger than the 

 other, and the smaller is seen in the group of faces at one of 

 its ends projecting from the side of the first. Such a crystal 

 is shown in figure 5. Of such an intergrowth there can be 

 two sorts, depending upon whether the twin individual has 

 been turned toward the front or the back of the one in normal 

 position. Figure 6 shows such a twin with No. II projecting 

 from the back of No. I. These twins appear sometimes in very 

 complex groups, one individual being twinned upon the first, a 

 third being in twin position in regard to the second, etc. Such 



* Zs. Kr., xliii, 582, 1907. fZs. Kr., xlv, 471, 1909. 



