198 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



A longitudinal section of anotliei' crystal (one-half natural size) is 

 shown in Fig. 5. It presents also an open skeleton appearance analogous 

 to that of Fig. 2. As seen in the figure the plates converge upwards on 

 either side of the longitudinal axis, meeting at an angle of approximately 

 35°. Like the previous case, it consists entirely of purely granular cr3-s- 

 tallized calcite with only a little mossy covering on the surfaces of the 

 l^lates. It is to be noticed here that the plates all converge upwards from 

 one extremity of the crystal to the other, and this, as will be remarked 

 later, is almost universally true even in the case of crystals, the external 

 form of which tapers off at both ends. 



Another transverse section (natural size) is shown in Fig 3. It is 

 like Fig. 2 in most respects, except that the square is elongated in one 

 direction and the diagonals meet in a central rib. Moreover while the 

 skeleton frame-work consists as before of crystallized calcite (left white in 

 ihe drawing), the intermediate spaces are partially filled up with a secon- 

 dary deposit of calcium carbonate, which is apparently amorphous, and 

 has been deposited in granular form and, too, in lines parallel to the 

 crystalline plates. This subsequent deposition, however, has not gone far, 

 and the general appearance is nearly as open as the one first described. 

 The outline of the crystal which yielded this section is shown in Fig. 7 

 (reduced one-half). As seen here it tapers gradually to the terminal edge, 

 forming a sharp extremitj^ The external form approximates to the true 

 crystalline form of the original crystal, but is somewhat more acute, as 

 shown by the edges of the plates exposed on the surfaces, and by the angle 

 at Avhich the plates within converge. 



In Fig 13 another section is given (natural size). This shows much 

 the same tessellated appearance, the structure being essentially the same as 

 in the others described, but the secondary deposition of amorphous calcium 

 carbonate has gone still further, so that as a whole it is more compact. 

 The skeleton ribs parallel to the sides and the diagonals are, however, 

 still very distinct and entirely crystalline. The form of the crystal which 

 gave this section is shown in Fig. 8 (one-half natural size). As seen here 

 it is an acute square pyramid, approximately conforming in outward form 

 to the internal structure. The surface is here no longer open and fretted, 



