100 L. V. Pinson — Artificial Lava-Flow 



by qualitative clieinical tests on material extracted fi'oin the 

 glass whic'li proved, in addition to the silica, tlie presence of 

 traces of iron and alumina and abundant lime and magnesia. 

 It is evident that a dolomite limestone was used in the making 

 of the glass. 



Mierostructure of Crystals. — When studied under the 

 microscope in powdered form it is found that the crystal 

 blades, ilhistrated in the photographic plate, are very far from 

 being the solid continuous crystals they appear. Close inspec- 

 tiou of them with a simple lens, as they lie in the glass, proves 

 them to have a parallel fibrous structure. The microscope 



Fig. 2. 





Figs. 1, 2. Skeleton crystals of diopside. 



shows them in length sections to consist of bundles of 

 extremely slender rods or fibers. These are sometimes closely 

 packed, sometimes separated by much more than their own 

 diameters. They are sui-rounded by, or are cemented together, 

 by the glass in which they lie. Sections across these fibrous 

 bundles, or blades, prove that to a great degree they are not 

 simple solid fibers, or rods, but are more or less hollow, or 

 skeleton-like in form, and that groups of them have a similar 

 crystallographic orientation. One of these is shown in the 

 adjoining fig. 1. The directions of extinction in this are 

 indicated by the broken arrow lines; these indicate the plane 

 of symmetry in the compound or skeleton crystal and it is 

 interesting that the directions of growth are along both pina- 

 coidal and prismatic faces. Another type is shown in fig. 2 ; 

 the growth here is along the clinopinacoid J, 010, and the 

 prismatic faces ; that along the othopinacoid being wanting. 

 A number of different patterns of growth were observed but 

 they are sufficiently illustrated by these examples. 



