110 



dried deposits of a puddle in the streets, when no apparent 

 change in the character of the materials deposited could be 

 noticed, and where there was certainly no interruption of depo- 

 sition. 



If the divisions of stratification, which I have thus pointed out, 

 be admitted, it is not improbable that in many cases of what are 

 now considered disturbed and tilted strata, it may be their nor- 

 mal condition. 



Mr. Girard read a letter from Miss M. H. Morris, of 

 Germantown, on the subject of the Seventeen Years' Lo- 

 cust. It contained many interesting facts connected with 

 the development and history of these insects. From her 

 study of them in 1817 and 1834, she had been led to the 

 following conclusions, which had been confirmed in her 

 mind by their appearance during the present year, namely : 



" That the larvse feed on the roots of certain species of trees 

 and shrubs on whose branches the parent has instinctively de- 

 posited them. That they are not apt to migrate. That when 

 groves or forests are cut away and the land cultivated for a series 

 of years, the larvse perish for want of food, and hence distinct 

 tribes have been found." 



The following extract also bears upon the question of a 

 distinct creation of tribes in different places. 



" My note book gives the following dates of their appearance 

 in various places. In 1845, they extended from Broad Moun- 

 tain, in Pennsylvania, far to the northwest in that State. The 

 northern portion of New York from Buffalo through the entire 

 length of Genesee county, in 1849. In Pennsylvania, the west 

 side of the Alleghany mountain, from the summit of Chestnut 

 Ridge into Ohio, in 1849. Northeast Pennsylvania and North 

 New Jersey, in 1850 ; South Pennsylvania, Maryland, and Vir- 

 ginia in 1851, and also in Georgia in 1851. 



" In most of these instances only a year intervenes between 

 the appearance of these tribes ; and as I have shown a few indi- 

 viduals always appear before and after the main body, the evi- 



