THINOLITIC TUFA. 199 



as in the others, but nearly smooth, except as it is covered with small wart- 

 like prominences. The color is a dark brown. The line in which the 

 section was cut is shown in the figure. 



Still another section is shown in Fig. 14 (natural size), and one 

 which marks a further degree of deposition of secondary calcium carbonate. 

 The crystal from which it was taken had a square form tapering slowly 

 upward, and the surface was covered with small mammillary prominences. 

 The skeleton of crystalline calcium carbonate is here nearly concealed by 

 the added amorphous material, and the outer portion consists of concentric 

 layers of the same substance. 



The exterior appearance of another crystal is shown in Fig. 4 (one- 

 half of natural size). As seen, it tapers slightly toward both extremities, 

 and it was cut longitudinally, in the idea that it might be a doubly ter- 

 minated crystal, but the structure lines all converged toward one end, 

 showing that, like most of the others, the growth was only in one direction. 

 As the surface indicates, the crystalline skeleton has been nearly filled up 

 with amorphous calcium carbonate. 



In addition to the sections given and others like them of large crys- 

 tals, numerous thin sections were also cut transverse and longitudinal to 

 smaller crystals. They revealed under the microscope the same points 

 which the microscopic examination of the larger sections showed— that is, 

 the presence of the same skeleton of crystallized calcium carbonate with 

 the concretionary depositions added to it. The calcite grains are large, 

 €ach one having a distinct rounded or elliptical outline, and they are 

 packed closely together, with a little brownish amorphous matter between 

 them. Many of them show the rhombohedral cleavage; others show a 

 crystalline nucleus which has apparently grown by the addition of further 

 crystalline matter. The secondary calcium carbonate has generally a 

 concentric or banded structure resembling some kinds of opal. 



These sections also show another point of interest; namely, the pres- 

 ence of groups of acicular crystals in parallel position filling more or less 

 completely the cavities in the skeleton structure, and sometimes projecting 

 into the cavities. These are seen in many cases, and are the general rule, 

 though sometimes absent; they are indicated magnified eight times in Fig. 



