ELY GREENSTONE. 143 



When the clearly recognizable characters of the spherulites, as shown 

 in the above illustrations, and the extent of the distribution of these 

 spherulitic greenstones are taken into consideration, it is with very great 

 surprise that one finds that the only recognition which the spherulites 

 have received was by N. H. Wincliell, who states, in his report," that the 

 surface of the rock is mottled by small areas of lighter color than the matrix 

 in which they lie, and refers to them as indicating an original amygdaloidal 

 or fragmental structure. Spherulites are now known to exist in the ancient 

 acid volcanics over various regions of the United States. They have also 

 been described from numerous localities where more recent acid lavas are 

 developed. Iddings well states the probable reason for the more frequent 

 occurrence of such crystallizations in acid than in basic lavas in the following 

 words : '' 



The greater frequencj' of lamination and localized crystallization in acid lavas 

 as compared with basic ones is a consequence of the generally greater viscosity of acid 

 lavas at the time of their eruption. The basic rocks have a considerablj^ lower melting 

 point and are much more liquid up to the temperature of solidification. Hence, 

 diffusion would take place more rapidly and the magna would be more homogeneous, 

 other things being equal. 



The spherulitic metabasalts or greenstones are extraordinarily abun- 

 dant in the Vermilion district. They have a very great development in the 

 adjacent portions of Ontario underlain by greenstones. The spherulitic 

 structure occurs in similar Huronian rocks in the Crystal Falls district 

 of Michigan, and is likewise developed in the rocks of the Menominee 

 district of Michigan. Mr. C. K. Leith reports the occurrence of similar 

 rocks in the Mesabi district of Minnesota. Considering- the extraordinarily 

 widespread development of tliis structure in the areas mentioned, one is 

 led to wonder at the fact that it is not present in similar rocks which have 

 a widespread occurrence in the Penokee-Gogebic district of Michigan and 

 Wisconsin, and in the Marquette district of Michigan. Furthermore, it 

 seems surprising that this structure should not exist in the somewhat 

 more recent metabasalts of the Keweenawan of the Lake Superior region, 

 and in the still more recent basalts of the Triassic of the Atlantic coast. 

 It seems highly probable that this spherulitic structure must exist, at least 



a Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. ly 1899, p. 253. 

 * Gpology of the Yellowstone National Park; Descriptive Geology, Petrology, and Paleontology, 

 by Hague, Iddings, and othert: Mon. U. S. Gcol. Survey Vol. XXXIl, Part II, 1899, p. 4?5. 



