130 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



Ill these rocks the ])Oi-phyi-itic characters are uuquestionalily due to 

 the production of secoudary phenocrysts of mica (muscovite) and calcite, 

 uot bv contact metaniorphisni but by dynamic action.'' 



It has not lieen found possible to determine definitely from a study of 

 the specimens, in many cases from widely separated exposures, on which 

 the above observations were made, whether the process which has taken 

 place iu the production of such rocks has been a combination of calcification 

 and silicification, or a process by which carbonate is being replaced by 

 silica or the reverse. The replacement of carbonate by silica, as shown by 

 Irving- and Van Hise,^has taken place extensively in the case of the ferru- 

 ginous carbonates of the Penokee-Grogebic and Marquette iron ranges of 

 Wisconsin and Michigan. The automorphic character of the carbonate 

 would seem to point toward calcification as the controlling process in the 

 Crystal Falls rocks. 



Though the presence of quartz as the last filling of the amygdaloidal 

 cavities points toward silicification as being the process which would 

 eventually predominate, it is most probable that both processes of calcifi- 

 cation and silicification are active ; but whether the one or the other is the 

 controlling one depends upon the depth of burial of the rocks which are 

 altering. 



This statement appears to be supported by the facts to be described in 

 the following pages. The following observations, which were made upon 

 sections taken from an ellipsoidally-parted basalt occurring on top of the 

 hills to the west of and overlooking Mansfield, illustrate the changes which 

 take place in the passage from the massive rock of the ellipsoids into the 

 schistose material of the matrix. The change is one of increasing altera- 

 tion. This alteration is largely one of carbonation followed by silicifica- 



' Metamorpbism of clastic feldspar in conglomerate schist, by J. E. Wolff: Bull. Mus. Couip. Zool., 

 Vol. XVI, 1891, pp. 173-18.S, Pis. I-XI. Cf. also Wolff on Green Mountains, Mon. U. S. Geol. Survey, Vol. 

 XXIII. 



Principles of Nortb American pre-Cambrian geology, by C. R. A'' an Hiso: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, Pt. 1, 1896, p. 692. 



Phases in the metamorpbism of the schists of Southern Berkshire, by W. H. Hobbs: Bull. Geol. 

 Soc. Am., Vol. IV, 1894, pp. 169-177. 



2 Origin of the ferruginous schists and iron ores of the Lake Superior region, by R. D. Irving: 

 Am. Jour. Sci., 3d ser., Vol. XXXII, 1886, pp. 255-272. 



The iron ores of the Penokee-Gogebic series of Michigan and Wisconsin, by C. R. Van Hise: 

 Am. Jour. Sci., 3d ser., Vol. XXXVII, 1889, pp. 32-48. 



The Penokee iron-be.aring series of Michigan and Wisconsin, by R. D. Irving and 0. R. A^an Hise: 

 Tenth Ann. Rept. U. S. Geol. Survey, 1889, pp. 341-507 ; Men., Vol. XIX, 1892, pp. 254-257. 



