TUE IGNEUUa KOGKS. 517 



of maf^nietito. Tlie nonamyydaloids are like the amygdaloids except that 

 thev lark aiiivii-dules. 



The only essential difference between these sheet eruptives and those 

 occiUTing- in dikes is in their structure, which in the former rocks is por- 

 phyritic and in the latter ophitic. 



The chlorite-schists interbedded with the iron-formation rocks at the 

 cut in the Chicago and Northwestern Railway are in all respects like the 

 chlorite-schists that have been dcscrilx'd as forming dikes. 



THE TUFFS. 



The tuff de])osits among the pre-Clarksbui'g rocks are not widely 

 spread. So far as known they occur only at two localities, where they 

 are very closely associated with knobs of greenstone. One of these is the 

 western end of the knob near the east quarter post of sec. 20, T. 47 N., 

 R. 27 W. (Atlas Sheet XXVI), and the other is the western end of the 

 northern of the two hills, near the east quarter post of sec. 4 (Atlas Sheet 

 XXV), in the same township. The relations of the tuffs to the massive 

 rocks are not clear at eitlier of these places. In the first locality, how- 

 ever, the tuffaceous beds are cut by large dikes of greenstone, similar to 

 the rock that forms the main body of the hill. The recognition of these 

 rocks as fragmental eruptives is due to microscopic evidence and to the 

 congh)meratic asjjcct of their weathered surfaces. Light-gray, pebble-like 

 areas appear in a schistose green matrix containing small fragments of 

 altered feldspar and pieces of green schists. 



The tuft's are composed of exactly the same minerals as are found 

 in the schistose dike-rocks and other greenstones, but their structure is 

 obscurely tuffaceous. Under the microscope fragments of plagioclase may 

 be seen distributed thickly among the other rock components, and large^ 

 rounded and irregular fragments of a fine-grained diabase may be detected 

 here and there embedded in a schistose, hornblendic, and chloritic ground- 

 mass. These tuffs are not unlike those occurring in the Basement Conqjlex. 

 They differ from the Kitchi schists mainly in the manner and degree of 

 their alteration. 



