226 RECORD OF SCIENCE FOR 1880. 



et:'., liave been formed from eruptives composed of olivine aud bronzite, 

 similar to rocks now found unaltered in their vicinity.* 



106. The origin of the ferruginous schists and iron ores of the Lake 

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

 paper on this subject, Irving discusses the several theories which have 

 been held and gives a summary of the results of a detailed study of 

 the ores aud 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, and 

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

 in some places the silicifyiug waters have given rise to actinolitic 

 magnetite-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 

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

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

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

 fragmeutal 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, aud the rocks there are now entirely mica schists 

 and slates, t 



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

 his previous statement that Sorby was the first to call attention to 

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

 uebolin made this observation several years before. 



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

 ples of Pressure Fluxion in 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 Phihulelphia, which are considered to be purely of erup- 

 tive origin, consistingof syenites, acid gabbros, trap granulites, 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. 2r)rj-27-2. 

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

 $ Vol. 31, pp. 225-226. 



