886 



• 'at tbe centre is about fifteen feet below the general level. I found the 

 bed to be composed of layers of blue clay and sand, coveriug at a slight 

 depth limerock. 



These sinks derive their name from the fact of their being collects 

 for the waters of the surrounding region, which are here absorbed ; no 

 outlets being discovered. The waters no duubt disappear in a cavern- 

 ous aperture of the limerock below. It is stated, on good authority, 

 that when the large sink is fil!ed,^a whirlpool may bo seen of sufficient 

 force to draw in rails floating near the vortex. 



Marks of Diluvial Action. — Upon the surface of the broad flat 

 ledges of limerock at Point au Peaux, was observed a beautiful exhi- 

 l>ition of the grooves and scratches supposed to be occasioned by attri- 

 tion of hard bodies moving in a strong: current. These have a uniform 

 direction, north, GO deg. east. In some instances the surface has been 

 worn to a fine polish, which imparts a beautiful light color to the stone, 

 and exhibits the markings as fresh in appearance as if done yesterday. 



A similar phenomenon is observable at Brest, and is a fine illustra- 

 tion of the course and moving power of the current. The surface, 

 about twenty square feet of which is exposed, is undulating and worn 

 into curves. Two directions are here "observe 1 of the furrows, on con- 

 tiguous portions of the same ledge; north 60 deg. east, and north 66 

 deg. west. 



Facts of this nature, which singly are of little importance, are noted 

 because they have a bearing upon the results of scientific inquiries; a 

 consequence which, though for a time unheeded and "darkly hid," tends 

 to throw clearer light upon the ultimate plan of useful economy. 



My acknowledgments are due to many persons in the above counties 

 for hosoitalities, and assistance rendered in the prosecution of the sui'vey. 



BEL A HUBBARD, 

 Assistant Geologist. 



Detroit, January 26, 1839. 



GLOSSARY, including most of the Geological terms used in these 

 Reports, from LyeW.i Geology, and other sources. 



Alluvial. The adjective of alluvium. 

 Alluvion. A synonym of alluvium. 



