BAROMETRIC PRESSURES ON THE GREAT LAKES 127 



It is broken up into only a few seiche areas. There are probably only three 

 important seiche areas in Lake Erie: (1) the area of depth greater than 20 

 fathoms in the eastern part of the lake; (2) the continuous area of more 

 than 10 fathoms depth extending from a point near the eastern end of the 

 lake (marked by three stars on plate 2) nearly to Sandusky and Point Pelee, 

 near the western end of the lake; and (3) the part of the western end of the 

 lake which is largely cut off from the main lake by Point Pelee and a chain 

 of islands including Pelee Island and Kelley's Island. The first-mentioned 

 seiche area is included within the second-mentioned. 



In contrast, consider Lake Michigan-Huron. It has a straggling area 

 bounded by extremely irregular shores interrupted once by the whole of the 

 lower peninsula of Michigan and again by the peninsula and islands which 

 in part cut off Georgian Bay and North Channel from the main portion of 

 the lake. Lake Michigan-Huron is probably broken up into at least 8 

 seiche areas, which are all of primary importance. The writer estimates 

 these areas, by examination of the Lake Survey charts, to be as enumerated 

 below: 



(1) That part of Lake Michigan south of latitude 44, in which the depth is more than 50 

 fathoms and less than 90. Note that the regularity of this area is broken in its middle 

 portion by a small oval, within which the depth is more than 90 fathoms, and another oval 

 near it, within which the depth is less than 50 fathoms. 



(2) That northern part of Lake Michigan extending from a point north of latitude 44 

 nearly as far north as the main entrance to Green Bay, in which the depth is more than 

 90 fathoms. 



(3) Green Bay. 



(4) That northeastern part of Lake Huron which is more than 50 fathoms deep, ex- 

 tending in a long area of gradually increasing width from latitude 46 and longitude 84 

 southeastward to latitude 441 and longitude 82. 



(5) North Channel, the long narrow area at the extreme north end of Lake Huron, 

 separated from the main portion of the lake by Manitoulin Island. 



(6) Georgian Bay. 



(7) The unnamed bay which is a southern extension of Lake Huron and which terminates 

 at Port Huron and the entrance of the St. Claire River. 



(8) Saginaw Bay. 



So far as there is a 7-hour seiche through the Strait of Mackinac, as dis- 

 cussed on pages 119-120, the whole of Lake Michigan-Huron is acting as a 

 single seiche area. 



In accordance with the contrast between Lake Erie and Lake Michigan- 

 Huron, to which attention has just been called, the observations indicate 

 that the seiches on Lake Erie are much larger in range, as a rule, than those 

 on Lake Michigan-Huron ; that the Lake Erie seiches persist with a relatively 

 large range for several days at a time; and that on Lake Michigan-Huron, 

 though very large seiches sometimes occur, such seiches are apparently 

 damped down to a small range, as a rule, after only a few oscillations. 

 The prevailing condition at each gage on Lake Michigan-Huron seems to be 

 that small seiches are in progress, seldom disappearing completely and 

 seldom large and regular enough to make it easy to detect the seiche periods. 



