548 PRINCIPLES OF STRATIGRAPHY 



Monroe dolomites in undisturbed horizontal position, but in front 

 of these at a level represented in the cliffs by bedded strata are 

 erosion stacks of the brecciated rock, carved from the cliff during a 

 period of higher level of the lakes. The position is such as to indi- 

 cate that these stacks are evidently a part of the rock stream, while 

 the cliffs behind the stacks are a part of the original cliff. In one 

 part of Mackinac Island the breccia is found to be underlain by 

 shales and thin limestones of Monroan (or Salinan?) age. (Figs. 

 117, 118.) 



That the rock stream represented a subaerial flow of the rocks 

 is shown by its character. Fine, rounded quartz grains, blown from 

 a distance, are incorporated in the mass. The age of this stream is 

 Lower Devonic, the Middle Devonic Onondaga strata enveloping and 

 enclosing it and partly incorporating it as a somewhat reworked 

 product. 



Residual Soils. 



The product of rock decay /// situ, whether of the nature of 

 laterite or kaolinite, pure or impure, not only furnishes material 

 for other agents to rework, but may also be recomposed in situ 

 without much or any disturbance. The same is true of crystalline 

 sand resulting from disintegration through insolation of granitic and 

 other coarse-grained crystalline rocks. If the upper layers are re- 

 worked by a transgressing sea or by wind or rivers, a perfect 

 gradation from the unaltered crystalline to the overlying stratified 

 rocks may be produced. Such a gradation is seen in the basal 

 arkose of the Lake Superior sandstone, where it rests on the pre- 

 Cambric peridotites. The indistinctness of the contact has led some 

 observers to regard it as of the igneous type. Residual soils of 

 limestone regions would have a sharper contact with the underlying 

 parent rock from which they are derived by solution. They consist 

 of the residual clay left after the removal of the lime in solution. 

 Such clay is rendered carbonaceous, if the old land surface presents 

 obstructed drainage, and swampy conditions prevail as on a pene- 

 plain. In extreme cases layers of coal of the terrestrial (humulith) 

 type may be formed. Eolian dust may be added, and the whole 

 subsequently reworked by a transgressing sea and covered by 

 marine deposits into which it will grade. Thus a complex deposit of 

 dark clay shales will be produced, the age of which ranges from the 

 period of exposure of the region to subaerial solution to the period 

 of resubmergence by the sea. Some of the mid-Palaeozoic black 



