THE IKON-BEAEING MEMBER. 201 



under the microscope, is furtlier shown by its ready solul)iht\' in caustic 

 alkalies. Nowhere does this chert present a concretionary or brecciated 

 appearance. 



The two constituent niineral.s occur sometimes in solid l^ands, alter- 

 nating with each other, and interstratified with these are other bands com- 

 posed in greater or less propoition of the chert and siderite. Sometimes 

 the bands of siderite contain a little chert (PI. xxi, Fig. 3), and the nearly 

 pure bands of chert frequently contain more or less of siderite, the indi- 

 viduals of this mineral generally being in a regular rhomljohedral form. In 

 the unaltered rocks the chert can no more be said to be a background in 

 which the siderite has crystallized than the reverse. Apparently, as a 

 result of changing conditions, regular alternations of siderite and chert and 

 vanous mixtures of the two have followed one after the other. Sometimes 

 the layers of solid siderite are of considerable thickness; tlie same is true 

 to a less degree of the cherty layers. These rocks appear to l)e in essen- 

 tially their original condition. If there has been anv change since they 

 were foi'med, there is no evidence of it, and for our present purposes they 

 must be regarded as original rocks. It is not impossilile that the>' have 

 undergone great changes, but if so we are unable to find an^" clew as to 

 their nature. 



From these apparently unaltered rocks of the first type there are gra- 

 dations to those of the first phase of the second type. The first alteration 

 to which these rocks are subject is an oxidation of the carbonate producing 

 brown hydrated hematite, red hematite, or magnetite. Very frequently the 

 decomposition of the iron carbonate has not changed the forms of the 

 original crystals, and thus leave the various oxides as perfect pseudonior])hs 

 after the iron carbonate. (PI. xxi, Fig. 4.) All phases of this change to each 

 of the iron oxides is exhibited by the various sections. Frequentlv upon 

 one side of the same section is unaltered iron carbonate, and ujion the other 

 side iron oxides alone, pseudomorphous after the iron carbonate. Between 

 the two are all stages of the change. The formation of magnetite pseudo- 

 morjjhs after the carbonate is much less common than that of the brown 

 hydrated hematite and the red hematite. In tlu- siderites which were more 

 nearly pure, this alteration, completely carried mit, iPiodiicis the lean 



