670 J. A. UDDEN COMPOSITION OF CLASTIC SEDIMENTS 



The composition of the deposits forming on the continental shelf are 

 presented in two tables, the first containing material collected from the 

 higher landward side of the shelf and the second containing samples 

 from the onter seaward slope of the shelf. It will be seen that the sam- 

 ples taken from the outer part of the shelf contain more coarse material 

 than the samples from nearer the land. This is no doubt due to the 

 presence of tidal undercurrents, which must be strongest on the outer 

 shoulder of the shelf. It appears that the irregularity of the topography 

 of the sea-bottom near the outer border of the shelf would also tend to 

 localize the currents and produce a greater variety of sediments. We 

 find in these analyses only one sample which is of fine composition. 



These samples show considerable variations in perfection of sorting. 

 A few are as well sorted as beach material, as in case of samples 135, 

 142, 148; but most samples more nearly resemble stream sediments. 

 Like these they have sometimes a secondary maximum. In a general 

 average of the chief ingredient and admixtures in both groups, more than 

 1 per cent is found in eight grades, more than one-tenth per cent occurs 

 in six coarse admixtu.res, and in five fine admixtures. A secondary 

 maximum occurs in the fourth grade of the coarse admixtures. 



List of Samples in Tahle 15 



134. Mud and fine sand, about one-tenth in all organic, from the upper slope 



of the continental shelf. The analysis applies to the inorganic part 

 only. Steamer Albatross; Station No. 871; latitude 40° 2' 54" north, 

 longitude 70° 23' 40" west. 



135. Coral sand, taken in channel between Cuba and Yucatan, 50 miles from 



land. Steamer Albatross; Station No. 2361; latitude 22° 8' 15" north, 

 longitude 86° 51' 15" west ; depth, 25 fathoms. January 30, 1885. 



136. Quartz sand and shell fragments, on continental shelf, SO miles south 



from Florida coast. The sample contains mostly quartz sand. Steamer 

 Albatross; Station No. 2369; latitude 55° 1' 0" north, longitude 160° 

 12' 0" west ; depth, 26 fathoms. February 7, 1885. 



137. Coarse gray sand and gravel, containing white and black sand and mica, 



east of the south end of Labrador. Steamer Trenton; Station No. 

 3522; latitude 57° 58' 0" north, longitude 56° 34' 0" west; depth, 41 

 fathoms. August 4, 1893. 



138. Green mud and fine sand, about one-third organic, taken on the conti- 



nental shelf. Steamer Albatross; Station No. 2249 ; latitude 40° 11' 00" 

 north, longitude 69° 52' 0" west; depth, 53 fathoms. September 27, 

 1884. 



139. Green mud, about one-seventh organic, taken above the edge of the con- 



tinental shelf, 70 miles from land, south of Massachusetts. Steamer 

 Albatross; Station No. 2240; depth, 44 fathoms; sample taken in 1884; 

 latitude 57° 0' 0" north, longitude 153° 18' 0" west. 



