484 PRINCIPLES OF STRATIGRAPHY 



well as the character of the rock, it must be assumed that the 

 regions were not open sea, but a coast lagoon or perhaps a marginal 

 epicontinental sea, laid bare to some extent at low tide with the 

 formation of extensive mud flats on which were stranded animals 

 and plants drifted in from the sea, and washed from the land. The 

 fact that the lower side of the fossils is generally better preserved 

 than the upper shows that the organisms were partly embedded and 

 corroded on the exposed surfaces by the acids generated on the 

 mud flats from the decaying organic matter. Had the water been 

 deep, the carcasses of the Ichthyosaurians, etc., could not have been 

 stranded in the mud, but would probably have continued to float 

 until they were cast on the shore or until decay had brought 

 about the dissociation of the skeletal elements, which would then 

 become scattered on the bottom. Instead of this, not only are they 

 intact, but the skin of the Ichthyosaurians has been found as a car- 

 bonaceous film surrounding the skeleton in its normal relationship, 

 besides these saurians numerous well preserved Pentacrini are 

 found which were carried into those water bodies attached to drift- 

 wood. Gastropods, worms, cephalopods, and crustaceans also occur, 

 and fish are likewise common and well preserved. Land plants, 

 conifers, and cycads abound, and land animals, especially insects 

 and pterosaurs, also occur. The driftwood is commonly trans- 

 formed into jet, by the secondary enrichment of the decaying wood 

 by bituminous matter from the mud. Besides these macroscopic 

 remains, the shale abounds in microscopic fossils of sponges ( ?) 

 (Phymatoderma), Foraminifera, coccoliths, and diatoms ( ?). Other 

 black shales of a similar origin are probably found in the Devonic 

 Ohio black shale with its rich fish fauna, and the L'pper Devonic 

 black shales of New York. The Marcellus shale of New York has 

 already been referred to as a similar sapropelargillyte, but formed 

 in the lagoons behind the Onondaga coral reef. The oil shales of 

 Australia, with the rhizomes of Glossopteris, the so-called "vertebra- 

 ria" often replaced by or transformed into jet, also belong here. 

 Finally, it must be emphasized that black shales also are formed 

 by the decay of land and swamp plants and that these, therefore, 

 belong more truly with the true coals and other humuliths. (See 

 page 513-) 



Sapropelcalcilyths, Sapropelsilicilytlis, and Sapropelferrilyths. 



Bituminous or asphaltic limestones are formed when lime sands 

 or muds or organic calcipelytes are deposited along with much 



