THE GEOLOGICAL SECTION OF MICHIGAN ' 397 



12. Trenton limestone. — Under this head have been grouped, as 

 appears from Foster & Whitney's map, and the text by them (p. 140, 

 Hall and others) equivalents to the Chazy, Birdseye, Black River, 

 ana Trenton in its narrower sense of New York. We are thus includ- 

 ing all the Mohawkian and the Chazy, the lower half of Schuchert's 

 Ordovicic. Grabau would include all up to the Black River in 

 his Chazyan or Middle Ordovician, and would also combine the 

 Trenton with the overlying Utica. In a general way it is what 

 Bigsby referred to (1823, pp. 195-96) as the "limestone of St. Joseph." 

 He refers to its typical exposures on St. Joseph Island and figures 

 characteristic fauna. He also gives an excellent lithological descrip- 

 tion, mentioning the characteristic "Birdseye," or as he calls it 

 "knotty," texture of some parts. 



Pleading that neither at top nor bottom do our dividing lines 

 exactly agree in time with the New York column, Grabau would 

 suggest a local name like Escanaba. But it is entirely unlikely that 

 the dividing lines are exactly the same at the two ends of the Upper 

 Peninsula' — that is, on the Escanaba and St. Mary's rivers where 

 alone it has been, or can be, studied. Still less likely is it that where 

 it has been struck in deep wells at the other end of the state, it is the 

 same. Yet all over this vast area the line of change to black shale 

 (Utica) from limestone or dolomite is well marked and of practical 

 importance. It probably represents not very far from the same 

 time. In fact, why should not a change in sedimentation at this 

 point be due to diastrophism involving an instantaneous or simul- 

 taneous change over a wide area, a general retreat of ocean due to a 

 large drop in its bottom somewhere, both shallowing the sea and 

 exposing the land to renewed erosion and so muddying the waters ." 



For subdivisions we may use the Green Bay wells: 



Galena limestone, crystalline, granular '. 83 



Limestone, fossiliferous 55 ft., white 8 ft., dark 9 ft 72 



Alternating blue and brown, crystalline, granular. With the dark base 



compare the Wisconsin oil rock 225 



Sandy limestones, "quartz" 6 ft., limestone 44 ft., quartz i ft., limestone 



24 ft., compare quartz sandstone at Marinette at 260-275 ft 75 



Wisconsin Trenton (Platteville ?), blue shale and limestone 41 



Blue shale 4 ft., black limestone 141 ft., limestone 19 ft., blue shale 4 ft.' 



I By a numerical slip in the Annual Report for 1903, p. 132, it is given as 41 ft. 



