226 RECORD OF SCIENCE FOR 1886. 



et^., have be. u formed froiueriiptives coiuposed of olivine aud bronzite, 

 similar to rooks now found unaltered iu their vicinity.* 



lOG. The origiu of the ferruginous schists and iron ores of the Liike 

 Superior district has long been a subject of controversy. In a receut 

 })^aper on this subject, Irving discusses the several theories which have 

 beea held and gives a summary of the results of a detaiied study of 

 the ores and schists into which they grade. The general conclusions 

 arrived at are as follows : In its original condition, the ore-bearing beds 

 consisted of a series of thinly bedded more or less highly ferriferous 

 carbonates, interstratified with and grading into carbonaceous shales 

 closely simulating the beds of carbonates in the coal measures. By a 

 process of silicification part of the siderite in these beds was broken up 

 and replaced by silica and the iron segregated into seams, layers, aud 

 impregnations in a more highly oxidized condition, but it is thought that 

 in some places the silicifying waters have given rise to actinolitic 

 mague'tite-schists and intermediate products. At some points the sid- 

 eritic constituent in the original beds appears to have been oxidized in 

 place, but the larger hematitic deposits are unquestionably secondary. t 



107. Van Hise, in a paper on the origin of the mica schists and black 

 mica slates of the Penokee-Gogebic iron-bearing series, gives an ac- 

 count of their lithologic characteristics, and discusses the mode of 

 their formation from clastic materials. Followed along the strike, the 

 quartzites, slates, and graywackes are found to change through biotic 

 aud chloritic graywackes to mica schists, forming the main mass of the 

 formation, by alteration of the feldspar and biotite to muscovite with 

 separation of silica ; " the result being a production from a completely 

 fragmental rock by a metasomatic change, of one presenting every 

 appearance of a complete original crystallization, and which would or- 

 dinarily be classed as a genuine crystalline schist." In the western 

 part of the district the feldspathic constituent was apparently in 

 greater proportion, and the rocks there are now entirely mica schists 

 and slates, t 



108. In a note to the American Journal of Science, § Irving corrects 

 his ])revious statement that Sorby was the first to call attention to 

 secondary enlargement of quartz grains in rocks, and states that Tor- 

 nebolin made this observation several years before. 



109. In a paper before the British Association, entitled " Some Exam- 

 ples of Pressure Fluxion ia Pennsylvania," Lewis describes his studies 

 upon some localities where evidences of this phenomena is found. The 

 principal one is the belt of Laurentian rocks crossing the Schuylkill 

 20 miles above Phihidelphia, which are considered to be purely of erup- 

 tiveorigin,cousistingof syenites, acid gabbros, trap grauulites, and other 



^ U. S. Geol. Survey, Bulletin (No. 2 ?), vol. 4, pp. 615-688, aud 4 plates. 

 t Am. Jour. Sci., in, vol. 32, pp. 25.')-272. 

 t Am. Jour. Sci., in, vol. 31, pp. 453-459. 

 § Vol. 31, pp. 225-226. 



