262 r. E. WRIGHT OBSIDIAN FROM ICELAND 



A comparison of these analyses, and especially of the norms computed 

 from them according to the methods proposed by Cross, Iddings, Pirsson, 

 and Wasliington, shows that they are all of the same general character. 

 The Icelandic rocks contain a slightly larger amount of femic minerals 

 than the other rocks, but the amount is not sufficient to place them in 

 another class. It is interesting also to note that the analysis 2, by 

 Bunsen, in 1851, agrees fairly well with the modern analysis 1 of the same 

 rock. In the legend of the table of analyses the symbols, according to 

 the quantitatiye classification of rocks proposed by Cross, Iddings, Pirs- 

 son, and Washing-ton, are given. These symbols show that in nearly 

 every case the rocks are located near the boundaries of the various sub- 

 divisions of the quantitative system. 



If the obsidian of Hrafntinnuhryggiir had crystallized under the 

 jjhysico-cheniical conditions of a deep-seated rock, the mineral composition 

 would probably have been : Quartz, about 40 per cent : orthoclase, about 

 16 per cent; oligoclase of average composition, AbgAnj, about 35 per cent 

 (some of the albite substance would probably enter into solid solution 

 with the orthoclase, Ijut to what extent can not be predicted because of 

 lack of information regarding these physico-chemical systems) ; aluminous 

 amphibole. 6 per cent; titaniferous magnetite, 2 per cent, and a little 

 apatite. The salts, XagCL and NaoSO^, indicated in the norm would 

 probably l)e carried away in solution as part of the more soluble portion/ 

 of the magma. It is, moreover, evident that the sodium would not be 

 the only base in combination with these acid radicles as postulated in the 

 norm. On the whole, experience has shown that in this persalane class 

 of rocks the modal composition is not greatly different from the computed 

 normative composition, 



SPHERULITES AND LITHOPHYSM 



Attention has been given in the preceding paragraphs to the intimate 

 relation between spherulites and lithophysae and to the several different 

 hypotheses which have been offered to explain the development of litho- 

 pliysffi. In the Hrafntinnuhryggur oljsidian all possible gradations oc- 

 cur between typical, compact, lithoid spherulites and typical lithophysae 

 with walls lined with minute crystallites, water-clear and very fragile. 

 Pumiceous structures are also of common occurrence, but they are usually 

 confined to definite bands and patches which alternate with streaks of 

 black obsidian glass containing occasional large vesicular cavities. In 

 the case of a wide band of glass these large cavities are more abundant 

 in the immediate vicinity of the pumiceous layers and virtually disappear 

 within a few centimeters. The gas cavities in both the pumiceous and 



