188 MICHIGAN ACADEMY OF SCIENCE. 



whose augite is more altered, which may be considered as similar dykes 

 of diabase, but coeval with some later part of the iron bearing series. 

 Thirdly, we have masses of similar rock which is usually called diorite, 

 along the cleavage or stratification of the iron bearing sediments. They 

 are not infrequently the foot-wall of bodies of iron ore. In them the 

 augite is generally very largely changed and sometimes instead of being 

 a granular mass of hornblende and feldspar, popularly known as diorite, 

 they become more uniform in color (greenstone) or are crushed and 

 changed still further so as- to be highly foliated and pass under the 

 names of chlorite schists, soap stone, paint rock, etc. These masses were 

 originally probably like the traps of New Jersey, and some of them in- 

 trusive like the Palisades of the Hudson and others effusive. The amount 

 of secondary alteration makes the problem of determining which a diffi- 

 cult one, not yet systematically solved. Finally, in connection with the 

 effusive sheets there are undoubtedly beds of tuff and sedimentary beds 

 composed of altered trap fragments. 1 think there is no doubt that all 

 these classes exist but some authors are inclined to see greenstone sedi- 

 ments everywhere and others to refer all doubtful cases to the igneous 

 rocks. Some of these traps undoubtedly have altered the adjacent rocks 

 as I have elsewhere remarked, and I believe a good deal of light will be 

 thrown upon this matter by study of the effects of baking and contact 

 metamorphism and the subsequent alterations. Little has been done in 

 this line yet. 



Another rather exceptional group of rocks are those in which some 

 of the ore deposits occur, which have been called by Wadsworth, jas- 

 pilite, — a rock largely composed of silica. It bears a strong resemblance 

 to some red ])orphyries or rhyolites and on account of tlmt and some 

 apparent igneous contact with schists. Dr. Wadsworth took it for an 

 igneous rock and thus spoke of the igneous origin of iron ores. Perhaps 

 all geologists are, however, now agreed that in the contacts spoken of, 

 the apparently stratified and sedimentary schists are really the igneous 

 rock and the jaspilite is the sedimentary. The greatest light U])on the 

 origin of the jasi)ilite has been thrown by Van Hise and Irving, who 

 worked into the problem from the Penokee-Gogebic Range of Wisconsin 

 and Michigan, where the stratigraphy is com])aratively simple. Their 

 theory, which is doubtless in large part true, is that the original rock 

 of the jasj)ilite was a clierty iron carbonate. In limestones deposits of 

 chert are very frequent and in the coal measures, beds of iron carbonate, 

 known as black band iron ore. are of a well known type, so that there is 

 nothing forced in the sup])osition made by Van Hise. We then have to 

 suppose that these beds have been altered by the action of water and 

 Van Hise has pointed out with great detail from Ihe results furnished 

 him by the engineers of many of the extensively exploited iron de])osits 

 that the iron ore rests in troughs made by impervious rocks, such as the 

 soap stone above mentioned or slate, down which the waters percolate. 

 The silica and carbonates have been removed and the iron ore concen- 

 trated. This concentration of iron ore bodies is, however, not confined 

 to the jaspilite formation but occurs in several different horizons and 

 particularly at the bottom of tlie formation whenever the structural 

 conditions are favorable; for instance, often replacing the Goodrich 

 Quartzite which lies at the base of one of the iron series. 



This brings us to another question, that of the subdivision of t lie iron 



