THE IKON-BEARING MEMBER. 195 



naked eye. More commonly, however, the use of the magnifying glass is 

 required. These main constituents are shown by the microscope to be in 

 most instances quartz, actinolite, hematite, and magnetite. The first of 

 these is occasionally lacking, but such phases are unusual. The four min- 

 erals vary greatly as to relative proportion, each in turn predominating 

 over the others, the lighter colored phases being of course the most highly 

 quartzose ones, while the darker kinds are richer in iron oxide, which 

 is often present in sufficient quantities to give the rock a more or less 

 distinct metallic luster. Except in certain thin seams, however, which 

 are at times even pure hematite and magnetite, the content is not so great 

 but that the powder of the rock is of a light color. In the more detailed 

 description given below it will be seen that in one unusual phase garnet is 

 a prominent constituent, and that chlorite and biotite occur frequently as 

 somewhat important alteration products. The lamination of these rocks is 

 usually more or less strongly marked by variations in color, tlie individual 

 laminae more commonly running from an insignificant thickness to about 

 one-fourth or one-half an inch, only occasionally exceeding the latter figure. 

 Parallel to the lamination there is generally a distinct tendency to cleavage. 

 Oblifiuely trans vei'se to this bedding cleavage is a jointing which renders it 

 exceedingly difficult to get from the ledge large sized pieces of the rock, 

 they usually coming out in small lozenge-shaped slabs. The large amount 

 of magnetite and hematite which nearly all phases of this rock contain 

 render it very noticeably heavy. The following are specific gravity deter- 

 minations made iqjon samples selected with a \aew to illustrating the sev- 

 eral phases of these actinolitic rocks: 305, 3"37, 342, 343, 3'46, 3"60, 3'53, 

 391, 431, 454, 5()1. The last three numbers represent thin seams unusu- 

 ally ricli in hematite and magnetite. The other numbers represent large 

 bodies of rock, the lowest one being given by one of the most quartzose 

 phases met with. The large amount of magnetite contained in the rocks 

 of this kind produces extraordinary attractions upon the magnetic needle. 

 In this connection reference should be made to the magnetic charts given 

 in the third volume of the Geology of Wisconsin, and in the atlas accom- 

 panying that volume.' ^^^lile the magnetic oxide of iron is the greatly 



\01urac Pis. xxiil, XXIV, XXV, xxvi, xxvii, xxviii, xxix, and xxx. Atlas Pis. xxiii, xxiv, 

 XXV, and XXVI. 



