192 STUDIES FOR STUDENTS 



bed of light-gray earth, shown by chemical analysis to be a residuum 

 of gypsum. 



The Monroe breccia of Michigan is considered by Hindshaw 1 

 to be of this type, although Lane 2 has classified it as a shoal breccia 

 and Grabau 3 as a rock-stream or rock-glacier breccia, at least in 

 its outcrops at Mackinac Island and the vicinity. As observed 

 by the writer, the breccia at this locality is a rubble, entirely with- 

 out bedding, of angular fragments set at all angles and varying in 

 size from gravel up to blocks 25 and 30 feet in length. Fragments 

 and matrix are of soft buff magnesian limestone, and the latter 

 is usually interstitial. Occasional seams of calcite and celestite 

 occur. In certain areas a zonal arrangement of material is seen 

 in the prevalence of chert, showing that the material of the breccia 

 has not been intimately mingled as in subaerial breccias whose 

 fragmentation is due to weathering. A still more conclusive negative 

 to such an origin is given in areas of rock quite undisturbed, such 

 as a block extending eastward along the cliffs for upward of 200 

 feet from the eastern border of the park at Mackinac. This 

 observation is confirmed by Rominger, 4 who states that "frequently 

 large rock-masses composed of a series of successive ledges which 

 have retained their original position to each other are scattered 

 through the breccia." The wide distribution of the breccia in 

 Michigan and Ohio precludes any local origin. The size of the 

 fragments and the absence of water-wear are considered incon- 

 sistent with a subaqueous origin. Along the eastern shore of the 

 island, however, numerous talus blocks show endostratic brec- 

 ciation. Laminae flexed and faulted and partially brecciated are 

 imbedded in a matrix of the same material and maintain more or 

 less of their original parallelism with each other and with the bedding 

 of the stratum. In the undisturbed block at the east of the park 

 at Mackinac sporadic fragments occur toward the base of massive 

 beds. These phenomena imply disturbed sedimentation, or sub- 



1 Michigan Geol. and Biol. Surv. Publ. 14, Geol. Ser. 11, pp. 206-7. 



2 A. C. Lane, Geol. Surv. Michigan, V, Part II, p. 27. 



3 A. W. Grabau, Science, N.S., XXV, 295-96; Principles of Stratigraphy, pp. 

 547-48. 



4 Carl Rominger, Geol. Surv. Michigan, I, Part VI, p. 27. 



