STUDIES FOR STUDENTS 181 



6. Coral breccia is intimately associated with stratified deposits 

 of coral mud and sand and pebbles. The reef contains stretches 

 of barren sand within the outer rim. The island is bordered by a 

 beach of sand and shows extensive tracts of sand in the interior. 

 Soundings disclose belts of sand with which the talus of the reef 

 must interfmger. Fine-grained limestones are forming in the 

 lagoon and in deeper offshore waters. 



7. Fragments may themselves be brecciform. Complex brec- 

 ciation occurs especially where fragments of the reef-rock breccia 

 are carried inland to form the island rock. 



8. The chief diagnostic of an ancient coral breccia is the presence 

 of the reef proved to be the work of corals by its fossils. Thus the 

 classification of the breccia of the St. Louis formation of south- 

 eastern Iowa and adjacent parts of Missouri and Illinois as a coral 

 breccia is held untenable by Bain 1 because of the absence of 

 reef-building corals. On the other hand, brecciated structures 

 connected with coral reefs are not necessarily of coral origin. 

 Associated with the Silurian reefs of Iowa are local breccias un- 

 questionably due to the later deformation of beds of limestone 

 accumulated upon the flanks of the coral mounds. 



Beach breccia. — On beaches where wave-action is inefficient 

 and angular blocks are supplied as from sea-cliffs, a deposit of 

 fragments so little worn as to be classed as breccia according to 

 prevailing usage may result under conditions of rapid submergence. 

 In all cases, however, more or less wave-wear will be found upon 

 the fragments, and the deposits, like other subaqueous deposits of 

 both angular and rounded material, should perhaps be termed 

 a breccia-conglomerate. 



The St. Louis breccia of southeastern Iowa, classified by Gordon 2 

 as reef breccia, is considered by Bain 3 a shore formation in which 

 blocks of limestone up to 4 feet in diameter were torn from their 

 beds and buried in sands apparently at the foot of a series of cliffs. 

 Savage 4 also finds evidence of vigorous wave-action and a close 



1 H. F. Bain, Iowa Geol. Surv., V, 150. 



2 C. H. Gordon, "On the Brecciated Character of the St. Louis Limestone," 

 Am. Naturalist, XXIV (1890), 305-13; Jour. Geol., Ill, 289 f. 



3 H. F. Bain, Iowa Geol. Surv., V, 150. 



4 T. E. Savage, Iowa Geol. Surv.,~XlI, 263 f. 



