Specific Gravity of Rocks, 313 



upon, without disturbing the materials. These dikes will in no 

 part be more than about one mile distant from the shore, and 

 when completed, will afford a shelter from the waves over seven 

 tenths of a square mile, having a depth of water of eighteen feet 

 at lowest springtides. That portion of the compass from E. to W. 

 round by the south, is protected by the formation of the shore. 



The whole work will constitute an aggregate mass of about 

 nine hundred thousand cubic yards of stone, the largest portion 

 of which is to be in pieces exceeding a ton weight each, and 

 although a smaller work than those of either Cherburg or Ply- 

 mouth, yet from the comparatively great distance from whence 

 the material is obtained, it is one of necessarily slow execution. 



The country for many miles around being a sandy alluvion, 

 the contractors for supplying the stone commenced with bringing 

 it from the Palisade rocks on the Hudson river ; but the tedious- 

 ness of the navigation, which consumed upon an average, ten 

 days for each trip, retarded the first season's operations very 

 much. Since then, the largest portion has been obtained from 

 quarries on the Delaware, between Wilmington and Crum creek, 

 a mile or two above Chester. Upwards of two hundred and 

 seventy-nine thousand tons have been already deposited, of which 

 eighty-one thousand were from the Hudson, and one hundred 

 and ninety-eight thousand from the Delaware, and the same 

 having been principally used in forming the upper end of the 

 first mentioned dike, it has afforded a shelter which was used by 

 the pilots, and by vessels engaged in the work, for protection 

 against the N. and N. E. gales, during the last two or three 

 months of the late working season. 



Specific Gravities of the Mocks, 



1 



From Christiana Creek below Wilmington, 



3,020 3-4 



C Hornblende or 

 (_ Greenstone. 



2 



" Brandywine, below the lowest mills, 



2,990 1-2 



do. 



3 



" Quarry ville, north of road to Wilmington, 



2,668 



do. 



4 



" do. near the river, south do. 



2,980 



do. 



5 



" Naaman's Creek, south do. 



2,688 



do. 



6 



" do. north, do. 



2,680 1-2 



do. 



7 



" Vicinity of Marcus Hook, north, do. 



2,751 1-2 



do. 



8 



" do. do. do. do. 



2,618 



do. 



9 



" Young's Quarry, Chester creek, do. do 



2,700 



Gneiss. 



10 



" Clark's do. do. do. 



2,764 1-2 



do. 



11 



" Hermis' do. do. do. 



Vqu I.— 40 



2,649 



do. 



* 



