STUDIES FOR STUDENTS 193 



aqueous glides, at the time of the deposit of the limestone, but the 

 main brecciation is taken to be of later date. 



There is also seen an association with folds which suggests either 

 a tectonic or a founder breccia. The underlying red and blue 

 shales seen in the rock bench about the island rise in places as 

 anticlines in the sea-cliffs. On the cliffs east of the fort at Mack- 

 inac, beneath a cornice of about 15 feet of horizontal massive beds, 

 appears a zone, 9 feet thick, of thin-bedded limestone, thoroughly 

 crackled, with unhealed seams, shattered, and in places with the 

 fragments rotated and displaced, but with the bedding still trace- 

 able to a large extent. In places low folds can be made out with 

 a height of 8 inches and width of some 4 feet, a deformation appar- 

 ently adequate to breccia te this brittle rock. 



The theory of founder presupposes the removal by solution of 

 beds of rock salt and gypsum underlying the red and blue shales 

 on which the limestone rests. These shales are not commingled 

 with the breccia of the limestone. If the breccia is due to founder, 

 it must be concluded that the soft clay shale yielded plastically to 

 unsupported pressure of overlying beds without mixing with the 

 fragments into which these beds were broken. In places small 

 chimneys of breccia penetrate the shales beneath, and the ledges of 

 breccia which rise in reefs above the country rock of shale in the 

 low ground of the north of the island may have a like relation. 



The grounds on which Hindshaw's theory of founder is sup- 

 ported are as follows: 1 The Monroe formation rests on the Salina, 

 which includes beds of salt, in places 800 feet in thickness, and of 

 anhydrite partially changed to gypsum. The Monroe itself con- 

 tains beds of anhydrite. In the Monroe beds and in the over- 

 lying Dundee limestone, which is also involved in the brecciation, 

 the circulation of ground-water is exceptionally active. Dis- 

 cordant and abnormal dips occur, accountable for by slumping due 

 to solution of the Salina beds about their outer edges. 



Cavern breccia is a local variety of founder breccia. Detached 

 masses fall from the roof and sides of a cave and accumulate on the 

 floor as breccia, which may come to fill nearly the entire space. 

 Cavern breccias may sometimes be recognized by their shape as 



1 Op. tit., pp. 206-7. 



