ANCIENT ROCK STREAMS 



547 



interpretation. Known examples of this type are the Siluro-De- 

 vonic breccias of Mackinac and vicinity in Michigan, the limestone 

 breccias of Gibraltar, the rubble drift of south England and the 

 rock streams of the Ural in the vicinity of the mines of Bakalsk. 



The Mackinac Limestone Breccia. (C,rabau-i8.) This is a 

 remarkable example of a breccia made up of large and small angu- 



FiG. 117. Diagram illustrating the relation of the brecciated limestone to the 

 bedded rocks at St. Ignace, Michigan. The stack is shown again in 

 Fig. 1x8. The dotted line represents the hypothetical former ex- 

 tent of the brecciated material. 



lar fragments of finely bedded upper Siluric (Monroan) limestones 

 and dolomites, derived from a still intact cliff of this limestone near 

 St. Ignace in the Upper Peninsula. The deposit is best seen in the 

 clififs of Mackinac Island, nearly the entire mass of which seems to 

 be composed of this rock, which must here have a thickness of be- 

 tween 200 and 300 feet or more. The fragments are of all sizes 



W.'TTvV '•' " 



Fig. 118. View of the ancient stack of brecciated limestone at Point St. 

 Ignace. The cliff of bedded strata is to the right of the railroad 

 (lower right-hand corner). (After Hobbs.) 



from that of a pinhead to blocks ten feet or more in diameter, and 

 their position in the breccia is such that the stratification lines of 

 the individual blocks dip in all directions. The distance to which 

 these blocks have been carried from the parent ledges is many 

 miles, and the width of this ancient stream is unknown. At St. 

 Ignace the high ground behind the beach consists of the stratified 



