Geology and Mineralogy. 



391 



individual crystal is roughly either that of a rectangular prism, 

 often with projecting edges and generally tapering off toward 

 the extremities (see figs. 1,4, 7, 8), or that of an acute square 

 or rhombic pyramid (fig. 9). Crystals of the former kind are 

 usually grouped in a more or less closely parallel position with 

 but little interlacing, the individual crystals having a length up 

 to 8 or 10 inches; when the crystals are smaller, however, they 

 interpenetrate each other and give the mass a reticulated aspect. 



The distinctly pyramidal crystals are generally small and project 

 from the mass in all directions. 



Of the true crystalline structure of the thinolite but little can 

 be definitely concluded from the external examination of the 

 specimens. It is only when sections have been cut, transverse 

 and longitudinal, that the form is revealed. The transverse sec- 

 tions show uniformly a system of square or rectangular ribs with 

 others in a diagonal position, intersecting at an angle of about 

 90°. In the porous kinds the section cut shows these ribs alone, 



