Fig. 145. 



SILICA. 



Fig. 146. Fig. 147. 



263 



Fig. 148. 



Figs. 130, 131, 132, 133, 136, 141, 142, 144, 147, 148; together with Figs. 149, 150, 151, 

 152, 153, 154, 155, 156, 157, 158, 159, 160*, and also the twins 140 and 161, from Mid- 

 dleville. 



Fig. 150. Fig. 151. 



Fig. 149. 



Fig. 152. 



Fig. 153. 



Fig. 154. 



Fig. 155. 



* Figures 157, 158, 159 and 160, are introduced on the authority of Dr. Horton. 



