GEOLOGICAL AND NATURAL HISTORY SURVEYS. 181 



about 15 feet a mile. Inland the formation thins more rapidly. The 

 thickening northward is attributed to the later upheaval of the north- 

 ern ranges. 



7. The upper or gray sanchtone. — This formation is " first noticed, 

 rising in hills, at a point not far distant from the River Ste. Marie 

 (at the Neebish Kapids) and southeast from Point Iroquois ; from this 

 point it stretches westerly in an elevated and very regular chain of 

 hills that are upon the coast as far as Tequamenon Bay; westerly 

 from which, the shape of the coast is such that these hills do not again 

 appear upon it until we reach that precipitous portion of the lake 

 coast known as the Pictured Rocks" (p. 42). Westerly from these 

 the ranges of hills formed of this sandstone stretch in a southwest- 

 erly direction, passing completely to the south of the primary trap 

 and metamorphic regions. This sandrock differs from the lower in 

 being more exclusively quartz, and al-o in ]la^■ing a south or south- 

 westerly dip, conformably with the overlying limestone, while the 

 lower sandrock dips toward the lake, and its outcropping edges are 

 covered by the upper and newer. The average thickness of the 

 Tipper sandrock as far west as tlie Pictured Rocks is thought to bo 

 about 700 feet. 



8. Sandy limeroek. — P'eebly rei)resented on Sailors Encampment 

 Island, it stretches westward, nearly along the middle line of the 

 Peninsula, with a Avidth of 10 to 15 mile^s, dipping uniformly to the 

 south-southeast. 



Above this succeed the lower limeroek and shales and the ftpper 

 limeroek group (Drummond Island and Mackinac limestone) de- 

 scribed in the third annual report. Covering the general surface, 

 lie the incoherent materials designated " Tertiary clays and sands." 



Doctor Houghton next proceeds, in the fourth annual report, to 

 consider " Economical geology." After referring to the valuable 

 building materials supplied b}' the granites and syenitic granites, and 

 the moderate promise of the red and gray sandstones for the same 

 purpose, he proceeds to the subject of " Minerals and mineral veins."* 

 He gives separate lists of the minerals occurring in the several forma- 

 tions before described, and then devotes 31 pages to " Mineral veins 

 of the trap, conglomerate, etc." 



Veins of date posterior to the uplift of the most northern range 

 are of frequent occurrence. The}^ not only traverse a portion of 

 the trap range, but also pass into the conglomci-ate, and sometimes 

 completely across the three sedimentary beds inmiediately above the 

 trap. They rarely vary more than 12° to 15° from a right angle to 

 the line of bearing of the sedimentary rocks, and in pursuing this 



' This portion of the report, amountin:,' to 'A?> pa'^es. was rr>nrinted entire in " The 

 Mineral Region of Lalte Superior." By .Jacob Hoiighton, Jr., pp. 36-79. EuEfalo, 1846. 



