HELDERBERG GROUP. 



31 



whether the boring has passed the water-Hme and entered the 

 Onondaga or Niagara group or not. Rocks exactly similar to those 

 brought up from the deepest portion of the drill hole are found in 

 the superficial beds of the quarries near by, and acicular lime- 

 stones, not distinguishable from those mixed with the upper 

 brecciated limestones, come up from a depth of over 200 feet. 



We learn by the borings that no gypsiferous shales are found in 

 the strata underlying Monroe, and that for several hundred feet 

 downward limestones of dolomitic character alternately follow each 

 other ; but we have no guiding rule by which to learn exactly 

 where we are. 



Among the exposures of the water-lime group, the quarries of 

 Gibraltar, situated about four miles south of the Trenton quarries, 

 are the most northern. There this lower rock series comes to the 

 surface in the bed of the creek where it enters the lake near Gibral- 

 tar, and west of the village, at the point crossed by the Michigan 

 Southern Railroad. The surface of the upper ledges is polished 

 by drift action. The rock is a somewhat absorbent, minutely crys- 

 talline dolomite of gray color and a laminated structure, some- 

 times deceivingly resembling an obsolete Stromatopora. The 

 beds are even, from i to 2 feet thick, answering a good purpose 

 as a building-stone. They frequently contain large, spherical, 

 concretionary masses, very hard and compact in the centre, but 

 successively becoming more and more porous toward their peri- 

 phery, which merges into the general rock mass without a defining 

 line separating the concretion from the plasma of the ledge. 

 About 8 feet of the rock beds are denuded in the quarry ; no 

 fossils were observed in the place. 



West of this locality, near Flat Rock, Huron River runs in 

 rapids over ledges which belong to the water-lime horizon, a 

 drab-colored, crystalline, somewhat porous, but hard dolomite, 

 with flinty concretions. Of fossils, I noticed casts of crinoid 

 joints, vegetable stems, and a small elongated body with email 

 surface, serrated on the edges, which can only be the remains 

 of a fish or crustacean. Similar corpuscles I found in the brec- 

 ciated limestones of Point-aux-Paux, a locality which will subse- 

 quently be described. The rock in the vicinity of Flat Rock 

 is covered by drift of considerable thickness, and does not come 

 near enough to the surface to be opened in quarries. At New- 



