392 W. S. Bayley — Spotted Rocks from Minnesota. 



Around the spots are clear zones corresponding to the raised 

 rims, mentioned as surrounding the hollow interiors of spots 

 on a weathered surface. These zones are sometimes composed 

 of grains and plates of epidote larger than those found in the 

 interior of the spots, either with or without calcite. When 

 epidote is present in the rims there is a scarcity of this min- 

 eral in the interior of the spots. This is the rule in those rocks 

 which contain a large amount of altered feldspar and chlorite. 

 In these feldspar, quartz, chlorite, a little bleached biotite, 

 little plates of hematite and grains of magnetite constitute 

 both the interior of the spot and the body of the rock. In 

 the spot the chlorite is better crystallized than elsewhere. The 

 rim contains only quartz, calcite and epidote, except on its 

 outer edge, where there is an accumulation of red oxide of 

 iron. In those cases in which the interior of the spots con- 

 tains a large amount of epidote, the exterior zone is compara- 

 tively free from this mineral, consisting essentially of quartz 

 and feldspar stained red by iron oxides. In both cases chlo- 

 rite is absent from the rim. 



Although each specimen of these spotted rocks, when ex- 

 amined under the microscope, presents features peculiar to 

 itself, it is unnecessary to describe them all. It is sufficient to 

 have called attention to their most striking characteristics, and 

 to have mentioned merely those features common to the class. 

 The incidental peculiarities due most probably to slight differ- 

 ences in the original composition of different parts of the beds 

 have been neglected. 



The occurrence of the spots in well defined beds lying be- 

 tween those which contain no spots, would at once lead to the 

 supposition that they owe their origin to the existence in these 

 beds of some substance, which was absent from those contain- 

 ing no spots. The shape of the spots and their groupings 

 suggest concretions. Epidote is not known to occur in such 

 forms in fragmental rocks, while concretions of calcite are 

 common. The existence of such calcite concretions in the 

 unaltered rocks of Pigeon Point is clearly established in the 

 one instance mentioned above (No. 11,461). Here we have a 

 quartzite differing in no wise from similar quartzites occurring 

 at a distance from the eruptive rocks, except in the possession 

 of calcite concretions. Nearer the eruptives, whether these 

 be granite or diabase, are other quartzites containing a little 

 more well individualized chlorite and large quantities of epi- 

 dote. This epidote is closely associated with calcite, the latter 

 increasing with the decrease of the former, and the two being 

 always bounded by the outlines common to concretionary 

 bodies in sedimentary rocks. As the result of a large number 

 of studies upon limestones which have been altered in the 



