FOSSIL REEFS: JURASSIC 441 



of these being dragon-flies. The pecuHar character of the fauna, 

 and the indications which it furnishes of having been stranded on 

 surfaces of calcareous mud, suggest that the lagoon was nearly dry 

 and flooded only during exceptionally high tides or during storms, 

 after which the water soon ran ofif again. ( Walther-91.) (Fig. 96.) 

 Some such conditions are represented to-day by the lagoons of Lil 

 and Mejt, in the Marquesas Archipelago, where the water is brackisK 

 and poor in organisms, while at Jabor and Jaluit the lagoons have 

 become fresh. (Agassiz-8 : j/, ^/j, 284.) Where abundant rain 



Fig. 96. A horseshoe crab (Limulus) and the marks of its death struggles, 

 showing that it was left stranded on the mud forming the lagoon 

 deposits. (After Walther.) 



water dissolves some of the limestones, the eolian sand is soon solidi- 

 fied by the redeposition of this lime in the interstices, as in the case 

 of the Bermuda sands. Moreover, some of this dissolved lime may 

 be reprecipitated in the lagoon, a process believed by Walther to 

 have taken place in the lagoons of the Solnhofen region. As a re- 

 sult of these and other chemical activities in the lagoons of these 

 reefs, we have not only the rapid and complete encasement of the 

 dead organisms and their perfect preservation in consequence, but 

 also the remarkable preservation of the muscle fibers of reptiles, 

 fishes, etc., which have been transformed into a rock containing 

 70% of Cd^^^X)^, 12% of CaCOa, 3.5% of CaSO,, 6.5% of CaFL, 

 3% of NagPO^ and 0.5% each of MgaP.Og and K3PO4, together 

 with small quantities of water and organic substances. 



The lagoon in which the Plattenkalke were formed was thus 

 apparently a great lifeless surface, on which the carcasses of land 

 and sea animals, cast there during storms or brought by the wind, 

 or, as in the case of the flying reptiles and the early bird, Archae- 

 opteryx, dropped there on the death of the animal, were so quickly 

 enclosed by the fine, chemically precipitated lime ooze that the usual 

 forces of destruction, scavengers, bacteria, etc., did not succeed in 

 destroying them or dissociating their parts. They were entombed in 

 a hermetic and antiseptic mud case, and thus preserved in all their 

 perfection of form and delicacy of structure which, until the re- 



