46 MICHIGAN SURVEY, 1908. 



may have, as a rule, passed farther south tlian at present. If these sug- 

 gestions are applied to the interpretation of the Glacial and post-Glacial 

 history of Isle Royale, the general relations will be about as follows: 

 Succeeding the disappearance of the ice was an arctic condition with 

 short summers and long winters, prevailing westerly winds, and severe 

 easterly or southeasterly moving storms. Such conditions as these 

 would influence the direction of late currents, wave action on the 

 beaches, and the source and movement of the lake drift, all of which 

 would greatly influence the biota. 



If the Glacial and post-Glacial adjustment of the permanent lows was 

 accompanied by severe storms, this would be a factor which would cer- 

 tainly influence the rate of formation and the distinctness of the beacb 

 lines, and it is not altogether improbable that a study of the well de- 

 veloped Nipissing beach, by the development of its spits and bars, may 

 furnish data regarding the lake currents and the prevailing winds. But 

 in order to interpret such records it will be necessary to formulate 

 criteria by means of which duration of a beach formation may be dis- 

 tinguished from one of less duration but due to more severe storms and 

 active currents. 



d. The Lake Storms and their Influence. The significance of lake 

 storms is of special interest on account of the bearing of the latter upon 

 the conditions of life upon the beach, and also upon the lake drift. That 

 they must be reckoned as an important factor in the post-Glacial repopu- 

 lation of Isle Royale is evident when we recall that during the 

 . life of the present fauna and flora the island has never been connected 

 with the mainland except by ice. Very fortunately the subject of lake 

 storms has been carefully investigated by Garriott ('03) because of its 

 influence upon navigation. 



The period of greatest seasonal frequency for severe storms ranges 

 from September to December, with a November maximum, while March 

 contains the greatest number of such storms for the remainder of the 

 year. The smallest number occur in June, July and August. 



There are several types of these storms, the most severe of which are 

 those of southwestern origin and which occur between October and May. 

 They are preceded by east and northeast winds which gradually become 

 a gale; but when once the storm center has passed the wind suddenly 

 shifts to the northwest and is an offshore wind from Canada. Such 

 storms are frequently followed by much snow and intense cold. During 

 the warmer months, storms from this direction are usually of tropical 

 origin. 



Less severe storms are those coming from the middle-west. These are 

 preceded by gales, first from the south and later from the east, and after 

 the passage of such a storm center the wind suddenly changes to the 

 northwest and finally finishes with clearing weather, or if in winter, 

 sometimes by a light snow. These storms are common at all seasons of 

 the year, but the most severe ones occur during the cold months. 



Storms from the northwest are seldom severe; they are preceded by 

 south or southwest winds, and after their passage the wind shifts to the 

 west and northwest and rapidly diminishes in velocity. In winter the 

 attending precipitation is generally light, in summer it is in the "form 

 of thunder storms, and the high winds in squalls from the southwest 



