MICHIGAN ACADEMY OF SCIENCE. 47 



THE CONTOUR OF THE SYLVANIA SANDROCK AND RELATED 

 STRATA IN THE DETROIT RIVER AREA. 



REV. THOMAS XATTtlESS. 



The beds exposed witliiii the new Livingstone Channel cofferdam at 

 Stony Ishmd, in Detroit River, do not at first sight offer any decisive 

 chie to their horizon such as the Devonian at the month of the Salt 

 Shaft at South Detroit affords; and have, perhaps as a consequence, 

 been mistaken by high authority for Later Silurian deposits, whereas 

 they are older than the Sylvania Sandrock. 



Stony Island itself forms a part of the wall of the coft'erdam, at 

 the west side of the dam. If we take this island as a centre we shall 

 find Sylvania sandrock S.W., in the extreme S.E. corner of Wayne 

 County, in Michigan ; S., on the foot of Bois Blanc island ; S.E., off 

 Bois Blanc, in the east channel of Detroit river and on Elliot's Point, 

 in the Dr. Green Shaft; S.E. x E., in the Caldwell Grove well; E. x 

 N.E., in the Sucker Creek test hole in Anderdon ; N.E., approaching 

 Lake St. Clair; N., in the Salt Shaft at South Detroit; and N.W., in 

 rock wells in AVyandotte. 



The area unwatered inside the cofferdam falls away toward the S.W., 

 S.E., N. E. ; W., S., E., and N. Within this area ^ the rock cutting 

 for the new channel that is being made to facilitate navigation, reveals 

 an anticlinal formation of Silurian rock. 



The cap of the anticline within the cut is abreast of Stony island, 

 the island itself being the highest jiart of what has been an insular up- 

 lift since somewhere in Silurian time, a part of the great Cincinnati 

 anticline. 



South from the cap of the Stony island anticline the strata bank 

 up against it at an angle to the south limit of a block that has faulted. 

 Southward of this the dip of the newer strata is much greater. In a dis- 

 tance of 300 paces from the point where the strata begin to bank up 

 against the higJiest elevation of the weathered anticlinal surface within 

 the cut an additional 30 feet of strata are picked up; and in a total 

 of 3,330 feet in length of exj)Osure there is a de])tli of 55 feet of rock 

 of higher horizon than the cap stratum of the anticline. 



At the south end of the area excavated within the dam a minimum 

 of five feet of rock required to be removed to get contract grade; at 

 the north end, 5,500 feet from south end, Oyo feet; and 21 feet at highest 

 elevation, about midway between. 



South of the coft'erdam, (and within the lower reach of its distance, 

 before the dam was built,) dredges worked to advantage. Still south- 

 ward, and west of Bois Blanc island, dredges went to grade without 

 assistance of the drill, working in a mud bottom. 



The conclusion is : this rapid southward dip of strata, traced now to 

 the close neighborhood of the Sylvania on the foot of Bois Blanc, and 

 on the west side of that island, contradicts the supposition advanced, 

 figured, and made the basis of conclusions in "Ncav Upper Siluric Fauna 



