284 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



considered here. The mechanical analysis on page 280 shows 31 per cent of 

 particles of silt, and 6.9 per cent of particles of clay size, 37.9 per cent of the 

 two sizes (see plate 94). The percentage of silt is about that usual in theTortu- 

 gas Lagoon deposits; but the percentage of particles of clay size is below the 

 average. The low percentage of the latter size is probably due to outwash 

 by tidal currents. Three samples were collected in the channel off the east 

 side of Garden Key. The clay ranged from 7.7 to 11. 5 per cent; silt from 17.9 

 to 36.3 per cent; silt and clay combined from 25.6 to 47.8 per cent. Where 

 the tidal currents are constricted, much fine material is washed away. The 

 maximum clay in any specimen from the lagoon is 15 per cent; this specimen 

 has 37.9 per cent of silt; the two sizes aggregating 52.9 per cent. The maxi- 

 mum silt in any specimen (one from Bird Key Harbor, depth 6 fathoms) is 

 56.2 per cent; the clay is 12.6 per cent; the two aggregating 68.8 per cent. In 

 the Tortugas Lagoon deposits there is definitely less material of clay size than 

 in any of the other lagoon deposits here considered; but the percentages of 

 silt and very fine sand seem fairly high. 



The percentage of MgC0 3 is 3.86 (partial analyses on page 270), dis- 

 tinctly higher than in the other lagoon specimens from Florida and the 

 Bahamas. This higher ratio of MgC0 3 is probably to be correlated with the 

 abundance of foraminifera and gorgonian spicules. 



Dr. Cushman furnishes the following list: 



No. 97. Near the obstruction buoy at southwest entrance to Fort Jefferson Channel, 

 Tortugas, Florida. Material includes fragmental calcium carbonate, frag- 

 ments and spicules of alcyonoids, fragments of pelecypod and gastropod shells, 

 fragments of worm-tubes, numerous ostracod valves, occasional echinoid 

 spines, and numerous foraminifera. 



Orbiculina adunca, few. Bigenerina nodosaria, few. 



Orbiculina compressa, few. Discorbina vilardeboana, few. 



Orbitolites marginalis, few. Truncatulina rosea, few. 



Peneroplis pertusus, few. Amphistegina lessoni, few. 



Quinqueloculina agglutinans, few. Nonionina scapha, few. 



Cornuspira involvens, one. Polystomella striatopunctata, few. 



Virgulina squamosa, few. Many Quinqueloculina, Trilocu- 



Textularia gramen, few. lina, Biloculina, etc. 



Textularia agglutinans, few. 



It is obvious that according to size the lagoon deposits of Florida repre- 

 sent three grades: (a) the finest, is represented by the specimen No. 91 from 

 between Loggerhead and Cudjoe Keys; (b) Marquesas Lagoon; (c) Tortugas 

 Lagoon. Grade (a) is the same as that found off the west side of Andros Island. 



SAMPLE FROM OFF KEY WEST. 



Specimen No. 100 (see plate 95) (J. B. Henderson, collector) was taken 

 from off Key West, south of Sand Key light, depth 60 fathoms. 



As this specimen was taken from the dredge, the mechanical and chemical 

 analyses are not so trustworthy as are those of the other specimens described. 

 (For mechanical analysis see page 280; graphic illustration, plate 94.) 

 The percentage of silt is 13; that of clay, 11.4; the two aggregate 24.4 per 



