THINOLITIC TUFA. 197 



sarfaces of the calcite grains. In other cases the outer surfaces are smooth 

 and rounded, and the unaided eye sees little of the structure except on a 

 cross-fracture ; in these instances, as Avill be more fully explained immedi- 

 ately, a deposition of calcium carbonate has filled up the skeleton form and 

 incrusted and smoothed over the surface. 



The specimens from tlie Domes represent still another type of the 

 tiiinolyte. The crystals here have uniformly an acute pyramidal form, and 

 are grouped in irregular, divergent positions. Their .surfaces are brownish 

 5 ellow in color and show little of the edges of the parallel plates conspic- 

 uous in the variety from the Marble Buttes They are, on the contrary, 

 nearly smooth, except when covered with watery excrescences, which in 

 fiorae cases are thickly clustered about the edges and extremities. One of 

 these crystals (natural size) is shown in Fig. 1 1, PI. XXXIII. On the fracture 

 this variety is found to be nearly as firm and compact as a fine-grained crys- 

 talline limestone ; in fact, the unaided eye would regard the whole as crys- 

 talline throughout. The color on the fracture is slightly yellowish white. 



Examination of sections of crystals. — In order to get at the true structure 

 of the crystals which have been described, it is necessary to resort to sec- 

 tions cut transversely and longitudinally; these reveal the form most clearly 

 and satisfactorily. A cross-section of a crystal like those first described — 

 the open, porous variety from Marble Buttes — is shown in Fig. 2 (natural 

 size). As seen in the figure it is made up of lines in position parallel to the 

 sides of a square prism, and in addition there are two sets of distinct diago- 

 nal lines intersecting at right angles to each other ; between these ribs are 

 open spaces. A closer examination of the specimen represented in Fig. 2 

 shows that the material consists of rhombohedral calcium carbonate, or cal- 

 cite, of a distinctly granular crystalline structure throughout. The whole 

 presents an open tessellated appearance. The external form of the crystal 

 which yielded Fig. 2 is shown reduced one-half in Fig. 6. The point at 

 which it was divided is indicated by a black line. The form is roughly that 

 of a square prism tapering slightly in both directions, but tlie external form 

 does not conform, in this respect, to the internal structure except at the 

 upper extremity. The irregular edges of the upwardly converging plates 

 are clearly shown in this figure. 



