HOBBS MINERALOGY OF WISCONSIN. 155 



Fig. 5. Crystal from Yellowsrone having apparently tetragonal sym- 

 metry from development in the direction of a principal axis. Figs. 5, 4, 

 5. Sections across a crystal of Galena from Yellowstone in the direction. 

 of the cleavage. These sections show an interior core which is outlined 

 by a line of tarnish and includes irregularly shaped cavities. This core 

 is surrounded by a later growth which is polysynthetically twinned with 

 the twinning plane the rhombic dodecahedron. Fig. 6. Crystal of Ga- 

 lena from Mineral Point showing polysyuthetic twin lamella? with the 

 twinning plane the rhombic dodecahedron. Fig. 7. Crystal of Galena 

 from Highland showing polysynthetic twin lamellaB with the twinning 

 plane the octahedron. 



PLATE 7. 



Crystals of cerussite, barite, malachite, and azurite from southern 



Wisconsin. 



Fig. 1. Crystal of cerussite of type 1 from Highland. The forms 

 present are: i, 2Pco ; a;, ^Poo ; y -^^Pao*; p, P; 6, ooPao ; ??;, ocP; and r, ooP3. 

 Fig. 2. Crystal of cerussite of type 2 from Highland. The forms pres- 

 ent are: p, P; m, ooP; a, ocPoo ; 6, ooPao ; i, 2Poo ; k, Px ; and x, ^Pco. 

 Fig. 3. Twinned crystal of cerussite of type 3 with the twinning plane 

 the fundamental prism. The types present are: t, 2Px ; x, iPx ; m, ooP; 

 and p, P. Fig. 4. Crystal of barite from the Linden mine, bounded by 



the forms c, OP; m, ooP; and d, ^Pao . Fig. 5. Crystal of Malachite from 

 Mineral Point. Figs. 6 and 7. Crystals of Azurite of domal habit. Fig 8. 

 Crystal of azurite of thick tabular habit. The forms present on these 



azurite crystals are: c, OP; a, ooPoo ; d, — Px ; c, — fPx *; 0, Px ; b, jP oo*; 

 n, iPx ; a, I Px *, e, | P2: and b, -fPI*. 



PLATE 8. 



Crystals of marcasite and pyrite from, southern Wisconsin and Galena, 



Illinois. 



Fig. 1. Crystals of marcasite of type 1 which has thin tabular habit 

 and is bounded by the planes; OP, ooP, and Px. Fig. la. Orthographic 

 projection on basal pinacoid of twinned crystal of type 1. Fig. 2. Lath- 

 shaped crystal of marcasite of type 2 bounded by the forms OP, ooPx , 

 Px , and ooP. Fig. 3. Stellate twin (cyclic flveling) of marcasite, consti- 

 tuting type 3. Fig. 4. Crystal of marcasite of type 4, bounded only by 

 ooP and OP. Fig, 5. Crystal of marcasite of type 5 possessing pseudo- 



