192 CARNEGIE INSTITUTION OF V/ASHINGTON. 



Origin of Beach-rock, hy Reginald A. Daly. 



Local cementation of beach-sand by calcium carbonate is very common in 

 Florida as well as in the tropical seas generally. The product may be called 

 "beach-rock." It includes some of the Florida coquina limestone. The 

 writer was invited to study the conditions under which this type of lithification 

 takes place. His field-work has been confined to the Tortugas, the keys and 

 mainland of Florida, and to six of the Samoan Islands. He found that beach- 

 rock occurs only on shores subject to heavy surf and directly fronted by detri- 

 tal flats which are shallow enough to be eroded by hurricane waves. The 

 upper Hmit of beach-rock is the sea-level at high spring tide; the lower limit is 

 usually, if not always, less than about 3 feet below sea-level at the low springs. 

 The original sand may be purely calcareous, but in many cases sand-grains, 

 cobbles, or large bowlders of lava or other siUceous material are important 

 constituents. An extreme case, wherein the original sand is nearly pure 

 olivine, was discovered in Tutuila, Samoa. 



Until the laboratory studies are completed, a definite conclusion as to the 

 origin of beach-rock is deferred, but, among the various hypotheses suggested 

 by the facts, one looks specially promising. Ordinary beach-sand is com- 

 paratively clean. Sand torn up from an offshore flat and piled on the shore 

 by a hurricane is necessarily charged with much organic matter, the subse- 

 quent decay of which would generate alkahes. It seems possible that the 

 alkahes would cause the interstitial deposition of cementing calcium carbonate 

 from the water circulating with each tide through the porous sand. To test 

 this hypothesis experimentally, offshore bottom material was buried at the 

 appropriate depth on the beach of Loggerhead Key, Tortugas. Typical beach- 

 rock was there formed, by the year 1912, in the sand piled up during the hurri- 

 cane of 1910. However, the hurricane of 1919 has torn up the buried material, 

 hindering an experimental test in the near future. It may be noted that the 

 hypothesis stated does not exclude the favorable consideration of other factors, 

 but their discussion is postponed until the preparation of the final report. 



While in Florida, the field facts bearing on the chemical deposition of 

 calcium carbonate in the form of mud were also studied, with the conclusion 

 that special attention should be devoted to the activity of eel-grass and other 

 sea-weeds as an indirect cause for the precipitation of the carbonate from the 

 saturated sea-water solution. 



The Geology of American Samoa, by Reginald A. Daly. 



The islands known as American Samoa consist of Tutuila with its satellites, 

 Aunuu, Cockscomb (Pola), and a number of islets; Tau, Olosega, and Ofu, 

 together forming the Manua group; and Rose Island. Excepting the last 

 named, which is an atoll, all the islands of Samoa, including British Samoa 

 (Upolu, Savaii, and their satelhtes), are volcanic. 



The geological investigation consisted of a somewhat detailed mapping of 

 Tutuila and reconnaissance of all the other volcanic islands of American 

 Samoa. Six days were spent at Upolu; lack of time and of transportation 

 prevented a visit to Savaii. The work was greatly facilitated by the use of 

 excellent unpubhshed contour maps of Tutuila and the Manua group. Photo- 

 graphic copies of the manuscript maps, on the scale of 1 to 40,000, were kindly 

 supphed by Dr. G. W. Littlehales, of the Hydrographic Office at Washington. 



The Samoan chain of volcanoes is strikingly hke the better-known Hawaiian 

 chain. The products and mechanism of eruption largely correspond in the 

 two groups. In principle Savaii is a close duplicate of its namesake, Hawaii; 

 Ofu and Olosega are remnants of a single island which has lost much of its sub- 

 stance by downfaulting, as at Molokai; Tutuila recalls the much dissected 



