THE CHICAGO ACADEMY OF SCIENCES. 



without effervescence. Apatite occurs in minute microscopic 

 crystals in nearly all the igneous rocks, and especially in gran- 

 ite. It is one of the earliest products of crystallization. In 

 comparison to the mass of the other mineral constituents of 

 the rock it is so small that in none of the specimens examined 

 does it occupy a more important place than that of an acces- 

 sory mineral. 



SULPHATES. 



THE SULPHATES are represented by two species in the nor- 

 mal hydrous group, namely, gypsum and melanterite. 



Gypsum. CaSO 4 +2H 2 O. Monoclinic. 



This mineral, which is the most abundant of all the sul- 

 phates, is represented by well formed crystals and groups of crys- 

 tals in the clay beds. A fine group now in the Field Columbian 

 Museum, was found by Mr. Duane Doty at Pullman, 111. A 

 photograph of a single crystal is given in pi. X. 



Fig. 21. 



It is a clear, transparent crystal 2^ inches long, which 

 began its crystallization around a string of .material that 

 has the appearance of some organic matter. Goniometric 

 measurements were possible and give iu A iTi 36; IIO A iTo 

 68. Fig. 21 represents a common twinned form in which 

 the twinning plane is parallel to the orthopinacoid. An- 

 other common twin is one parallel to the orthodome, and 

 when the pyramid planes are bent, the so-called swallow-tail 

 twin results. The groups in clay are stellate, radiated, and in 

 globular form. The pyramid and orthodome planes are often 

 rounded by repetition of these faces. The cleavage parallel to the 

 clinopinacoid (oio) is very perfect, so that this plane shows a 

 pearly lustre. Parallel to the orthodome the cleavage is so 

 perfect that crystals are often full of cracks in this direction, 

 and consequently present a fibrous appearance and a silky 



