HELDERBERG GROUP. 



29 



turn has, within the State of Michigan, never been found to exceed 

 8 or 10 feet, and in Ohio, also, it forms only a comparatively- 

 thin seam. No fossils are known to have been found in 

 it except the few fragmentary specimens which I mentioned, 

 so that a palseontological comparison with other beds is for- 

 bidden ; but considering its relative position between the upper 

 and lower divisions of the Helderberg group and its lithological 

 character, its equivalency with the Oriskany sandstone of New 

 York becomes very probable, an identification which was first 

 made by the Ohio geologists. A marked change in the nature of 

 the deposits above and below this geological horizon is perceptible. 

 The rock series below the sandstones which has been identified 

 with the water-lime group of New York, is altogether composed of 

 dolomites and contains entirely different fossils from those found 

 in the strata above the sand rocks, which have more of a true 

 limestone character, and rarely contain a high percentage of mag- 

 nesia. I have previously made the statement that this lower 

 water-lime division has a much greater surface extent within this 

 southeastern corner of Michigan than the higher beds. All the ex- 

 posures of Raisin River, in Plum Creek, belong to this lower rock 

 series ; the quarries in the townships La Salle, Ida, and Bedford 

 are worked in the same beds, and the outcrops on Point-aux-Paux, 

 at Gibraltar, in Swan Creek, and Stony Creek, all represent this 

 water-lime series. The upper strata of this group are frequently 

 but not always found in a brecciated condition, with fragments of 

 various ledges intermingled and re-cemented. The lower, non- 

 brecciated beds have evidently the same lithological characters as 

 the fragments composing the breccia. 



By what force the strata were shattered into fragments, it is 

 difficult to suggest, but the cause of the disturbance can not 

 have been a local one, confined to this district only. We find 

 this rock in brecciated condition all over the northwestern part 

 of Ohio and on the islands of Lake Erie. The same occurs 

 again in the equivalent beds of Mackinac, and in the exposures 

 of Goderich in Canada. 



A natural section through the whole thickness of the water- 

 lime group is nowhere in the district exposed, and the artificial 

 excavations by quarrying generally comprise vertically only a 

 small series of beds, and they are nearly always of one and the 



