SOUDAN FORMATION. 225 



and it is within this sjmcline that the deposits occur. Before the jaspers 

 were as intricately folded as at present they were intruded by dikes and 

 sheets of acid rocks similar in composition and general character to those 

 now outcropping on the islands and shores of Vermilion Lake. These 

 sheets of igneous rocks were intruded essentially parallel to the bedding of 

 the jasper, and were at varying horizons in the jasper, and hence are sepa- 

 rated by varying vertical distances. The intrusion of these sheets has thus 

 divided the iron formation, as it were, into a number of bands of different 

 thickness. The dikes which cut through the iron formation are of varying 

 trend, and these, as well as the intercalated sheets, were intruded in the 

 iron formation at various angles with the horizon. When, after their 

 intrusion, the rocks were folded, the intrusive sheets behaved essentially as 

 intercalated beds in the iron formation and were crumpled with it into close 

 synclines and anticlines. When the folding took place the brittle jaspers 

 accommodated themselves to the movement by fracturing, whereas the less 

 brittle eruptive rock- accommodated itself by shearing. 



Owing to the fractured character of the associated iron formation the 

 downward-percolating waters passed readily tln-ough it, but were stopped 

 and led along the relatively impervious acid igneous rocks. These were 

 thus intensely affected by the circulation of the water, which, bringing iron 

 in large quantity in solution, deposited considerable quantities of it in the 

 igneous rocks during their alteration. As a result of the action of the 

 percolating waters these rocks were intensely altered chemically. The 

 addition of iron rendered possible the formation of the chlorite, which is 

 not as a rule characteristic of the alteration of acid rocks. Now these 

 intrusives are essentially the same in general appearance as the soapstone 

 and paint rock derived at various places from the basic greenstones. From 

 the point of view of their influence in the formation of the iron-ore deposits 

 they are also absolutely identical with the above-mentioned soapstones, 

 originally of basic character, and they will be designated as soap rock and 

 paint rock, in accordance with the custom of the miners. 



Occurring in the way described, in the large central syncline of the 

 iron formation, these eruptive sheets have divided it into a number of small 

 synclines, each with an essentially impervious basement of soap rock. The 

 sheet of rock forming the impervious bottom of one trough forms the 

 impervious top to the next lower synclinal trough. Between these lie 



MON XLV — 03 15 



