and its Spherulitic Crystallization. 



103 



flow). Where one has a cavity the ones immediately adjacent 

 are apt to have the crack. In some places this crack has 

 opened quite widely and an ellipsoidal or lenticular cavity is 

 produced, bordered on either side by half spherulites extend- 

 ing into the fflass. The bearina; of tliese facts is discussed in a 

 later place. 



Another feature, quite similar to what is often seen in acid 

 volcanic glasses, is the presence of cracks in the glass immedi- 

 diately surrounding the spherulites. These commonly start 

 out from the edge of the spherulite, and especially from the 

 place where two tend to intersect, and then curve, sometimes 

 splitting and forming a Y. Usually after a short curve they 

 stop, but quite often they continue in a long cm-ve concentric 

 to the outer surface of several adjacent spherulites and removed 

 from it a small fraction of the total diameter of one of them 

 (see fig. 3). It is owing to these concentric cracks surrounding 

 them that the larger spherulites may be readily broken out of 

 the glass and then appear with a thin, smooth skin, like var- 

 nish, coating' them. 



Fig. 3. 



Fig. 3. Spherulites with cavities and cracks. 



Stony Material. — In the material represented in the collec- 

 tion there are some pieces of a white lithoidal or porcelain -like 

 appearance. They have a light greenish tone of color and a 

 deeper green by transmitted light. A number of small ves- 

 icles or gas pores are here and there seen in the material. 

 Under the microscope it is found to consist of glass filled with 

 microlites ; some of rather short, minute prisms of diopside 

 with inclined extinctions and masses of feathery and fern-like 

 forms of the same substance. There are also minute fibers 

 whose extinction appears parallel and whose exact nature is 

 doubtful. 



Obsidian. 



One of the most interesting features of the collection is the 

 presence of several specimens of jet black artificial obsidian, 

 appearing quite similar in color, luster, and fracture to the 

 natural obsidians from the Yellowstone Park, Mono Lake, 

 Ijipari Islands, and other well known localities. Flowage lines 



