84 On the occurrence of Enhydros. 



transparent. Some of the planes are striated, others are 

 covered with small pits. In such cases the corresponding 

 plane has similar striation or pits. These pits seem to be 

 the halves of air bubbles similar to those occurring in glass, 

 and that this is most probably the truth is proved by one 

 piece found, that shows unmistakable bubbles in the solid 

 chalcedony. 



The enhydros vary in size from 5 inches across to the 

 size of a split pea, containing fluid and a moveable bubble of 

 gas. In many cases they show no indication of a bubble 

 until unearthed for a few days. The bubbles also some- 

 times increase and decrease in a mysterious manner, and 

 occasionally the fluid gradually disappears altogether. 



The shells vary in thickness from \ inch to the thinness of 

 note paper, so thin, in fact, that they may be crushed 

 through holding them incautiously. 



The interior of. some of the shells is quite smooth, others 

 have drops of chalcedony on the inside, and a great many 

 are coated with, or quite filled by quartz crystals. These 

 crystals are generally minute, but sometimes \ inch in 

 diameter. They have grown so close together that the 

 prismatic planes are scarcely discernible, but the pyramidal 

 ones are perfect. 



The lai-ger enhydros are seldom coated with crystals, and 

 the fluid in such cases seems to be denser than in those 

 so coated. 



Is it not probable that the hardness of these stones might 

 be accounted for by their containing a small porportion of 

 alumina in combination, and — to account for the quartz 

 crystals — by supposing the solution, as soon as it became 

 imprisoned, gradually to have deposited its impurities and 

 then to have formed the pure quartz crystals ? 



In some of the cavities, in addition to the fluid there are 

 specks of dirt and clay that move about on turning the stone. 

 In one specimen hexagonal prisms of some mineral have 

 penetrated from one side of the shell to the other ; over these 

 a coating of silica has formed, subsequently the mineral has 

 decomposed, and the casts are now filled with white clay. 

 In another specimen there is a cavity not larger than a pin's 

 head containing a scarcely discernible moveable bubble. 



The following minerals occur in the granite about 100 

 yards south of the enhydros. 



Mica, black, white, and green, the latter in Stellate forms. 

 Chlorite, Garnet, Black Tourmaline, Chalcedony, Iron 



