22 New York State Museum 



erally recognizable in the lignite. The bituminous 

 coals are our common soft coals. A large proportion 

 of the Carboniferous (Pennsylvanian) coals are of this 

 nature and most of the younger (Mesozoic) coals. 

 Anthracite coal, our hard coal, is found where much 

 disturbance has taken place in the rocks and the bi- 

 tuminous coals have become metamorphosed. The 

 Carboniferous coals are the most important and most 

 extensive, forming the bulk of the coals of the world. 

 The other two coal- forming periods were during Cre- 

 taceous and Tertiary times. Cannel coal is generally 

 considered to be formed from fresh-water algae, also 

 to a large extent from spores and pollen grains of 

 higher plants that have blown into the bodies of water. 

 Hard parts of animals are also found. Algae, spores 

 of plants and animal tissues are probably the chief 

 source of petroleum. 



Consolidation of materials. Sediments, whether of 

 mechanical, chemical or organic origin, before they be- 

 come rock in the strict sense must in some way become 

 consolidated. 



Unconsolidated mechanical sediments include muds, 

 sands and gravels. Upon consolidation muds become 

 shales, lime-mud rocks etc. ; sands become sandstones 

 or, in the case of lime sands, limestones; gravels •be- 

 come conglomerates or breccias etc. The process of 

 consolidation is known as induration. Clastic sedi- 

 ments may become consolidated in various ways, but 

 pressure, cementation and recrystallization are among 

 the chief ways. The last-named process belongs with 

 the changes due to metamorphism and will be treated 

 under metamorphic rocks. Pressure alone may cause 

 induration, but probably in all cases of rocks thus 



