Ransome ] 



Eruptive Rocks of Point Botiita. 



explanation therefor. Professor Lawson,* in 1885, described the 

 so-called "concretionary structure" in the much altered basic erup- 

 tives of the Lake of the Woods region, but without discussing its 

 origin. G. H. Williams f in 1890 described and figured a spheroidal 

 structure in the aphanitic greenstone near Marquette, and pointed 

 out the identity of the forms with those described by Professor 

 Lawson. He shows that "they are in no sense concretionary," and 

 favors the hypothesis of Rothpletz,^ that they are due to breccia- 

 tion and rubbing caused by intense orographic pressure. Cole and 

 Gregory, § in the same year, saw in "the irregular shapes and 

 involuted surfaces of the diabase of Mt. Genevre the evidence of 

 lavas rolling over among themselves." Gregory, || in 1 891, describ- 

 ing the spheroidal diabase of the Fichtelgebirge, suggests that the 

 diabase cooled rapidly and contracted into spheroids, which were 

 then rolled over among themselves. In the present year Miss 

 Raising describing the spheroidal structure in the rocks of the 

 Lleyn, regards it as being produced by a spheroidal parting during 

 cooling, the spheroids being subsequently moved and modified by 

 a second influx of lava. 



It seems to the writer that none of the foregoing theories offer a 

 complete explanation of the structures described in this paper, 

 although the idea advanced by Messrs. Cole and Gregory appears 

 to afford the most suggestion. The close contiguity of the sphe- 

 roids or bales, the absence of any considerable amount of interstitial 

 material, and the striking evidence which they show of former plas- 

 ticity, are facts not to be reconciled with any theory calling for a 



* Report on the Geology of the Lake of the Woods Region, by Andrew C. 

 Lawson, M. A., Geological and Nat. Hist. Surv. of Canada, 1885, pp. 51-53. 



fThe Greenstone Schist Areas of the Menominee and Marquette Regions of 

 Michigan, by G. H. Williams, Bull. No. 62, U. S. Geol. Surv., Wash., 1890, 

 pp. 166-168. 



J Ueber Mechanische Gesteinsumwandlungen bei Hainichen in Sachsen, 

 part 2, Zeitschr. Deutsch. Geol. Gesell., Vol. 31, pp. 374-397. Cited by Wil- 

 liams, but not accessible to writer. 



gOn the Variolitic Rocks of Mont Genevre, Q. J. G. S., Vol. XLVI, p. 

 316. 



|| On the Variolitic Diabase of the Fichtelgebirge, ibid., Vol. XLVII, p. 60. 

 f Variolite of the Lleyn and Associated Volcanic Rocks, ibid., Vol. XLIX, 

 p. 152. 



