200 GEOLOGICAL HISTOEY OF LAKE LAHONTAN. 



15. These acicular crystals show uniformly extinction parallel to their 

 prismatic direction, and hence it seems clear that they must belong to an 

 orthometric system. It seems probable that they are aragonite. A chem- 

 ical examination of an uncovered slide gave results in accordance with this 

 suggestion. 



A section of one of the crystals from the Domes is shown in Fig. 

 12 magnified eight times. To the e3'e the broken crystal appeared to be 

 crystalline throughout; in the section, however, as examined vmder the 

 microscope, there is seen to be a crystalline frame- work made up of calcite 

 grains, filled in with amorphous matter, and in addition outer layers of 

 banded opal-like carbonate, so that it conforms in general to cases like 

 those before represented. The diagonal lines are here clearly developed, 

 and there are also rectangular lines more or less distinctly indicated. 

 These are illustrated somewhat obscurely in the figures. Other sections 

 showed essentially the same relations. 



Structure in dissected crystals. — As has been stated, the external form 

 of the thinolite crystals seldom gives the true crystalline foi-m. The process 

 of dissection, however, which has laid bare the skeleton-like ribs which 

 have been described, sometimes results in showing the true pyramidal form 

 of the original mineral. In such cases we may liave a series of skeleton 

 crystals, each a hollow pyramid as a cap to the one preceding. This is 

 shown in Fig. 9, which will explain itself, and again in Fig. 10 (both 

 natural size). In another case a mass of the calcareous tufa, showing little 

 structure, has its surface partially covered with pyramidal crystals an inch 

 in length. Each one was a skeleton crystal inclosing a pyramidal crystal, 

 and sometimes several crystals after the fashion of a nest of pill-boxes. 

 The outer surface of the crystals was incrusted with a moss-like covering, 

 often entirely hiding the form. Two of these are represented in Figs. 16 

 and 17 (natural size), and another in Fig. 29, Plate XXXIV. 



Before passing to the description of the next succeeding tufa deposit, 

 we may note that the external surface of the thinolite does not exhibit 

 evidence of having been weathered previous to the deposition of the den- 



