142 



the laminae are opened in the manner of a flaw or crack, 

 when not too wide, they admit the prismatic rays : see the 

 vpperface of middle figure and middle face of lower figure. 

 They are said to admit of double refraction by most minera- 

 logical writers. 



Fig. 1. is a darkish spot of clay or soil naturally in the 

 subject, and fig. 2. is the same seen a little duller through 

 the other face at the same time : but this is common to all 

 transparent substances. It is somewhat curious that the 

 clayey stripes or spots have a particular direction diagonally 

 to the acute angles. Spec. Grav. 2.2642 — 2.3117. Kirw. 

 and Haiiy. They are laminated, the laminae somewhat flexi- 

 ble; easily separated. 



Hardness, — yields easily to the finger nail. 



The lottom figure shows three crystals mixed together, 

 and forming what is commonly called a made. 



TAB. LXVIIL 



Is a variety from Bedfordshire, sent me by favour of the 

 Rev. T. O. Marsh, showing a tendency of the laminae to se- 

 parate and bend, which they will generally do in the longitu- 

 dinal direction. Thus, a plate of gypsum will be found ta 

 break less readily in this direction, always bending before 

 it breaks, and then generally ruggedly. In the other direc- 

 tions, it is either glassy or foliaceous. When these crystal- 

 lizations spread like the lower figures, they are commonly 

 called Lions' paws. Crystallized selenites are the moon- 

 stones of Gesner and Agricola. See Plott's Oxfordshire, 

 p. 81. 2 



