JET; BLACK SHALES 483 



are perhaps the most important deposits are found in the Liassic 

 rocks (zone of Ammonites serpentinus) of Yorkshire, England, 

 being especially obtained near Whitby, where it is mined and 

 wrought into all sorts of ornaments and toys, and, together with 

 the famous ammonites of the same formation, sold as native curiosi- 

 ties. The jet here occurs in thin lenticular masses between the 

 layers of the hard bituminous shale (sapropelargillyte) and occa- 

 sionally shows under the microscope the structure of coniferous 

 wood, referred to araucarians. Scales of fish and other organisms 

 of jet rock are frequently impregnated with this bituminous matter, 

 which may replace the original tissues. Drops of liquid bitumen are 

 also found in the cavities of some of the fossils. Petroleum and 

 inflammable gases are likewise associated with jet deposits and iron 

 pyrite occurs, often replacing the fossils. The Lias of Wiirttem- 

 berg in Germany also furnishes jet, especially the Posidonia shale, 

 which is of the age of the Whitby beds. Of the same age is the 

 shale furnishing the original deposits on the river Gagas in ancient 

 Lycia, Asia Minor. Jet has also been obtained from Tertiary de- 

 posits. 



Jet is characterized by its hardness, which is greater than that 

 of asphalt, its conchoidal fracture, and by the fact that it is less 

 brittle than anthracite, and is susceptible of a high polish. Its com- 

 mon association with driftwood leads to the supposition that its 

 chief source is carbonized v/ood enriched by secondary impregnation 

 with bituminous matter obtained from sapropelite. Analysis of the 

 jet from Holznaden, Wiirttemberg, gave C 71.0%, H 7.7%, O 

 23.3%, N trace, S trace, Ash 0.9-2.9%. 



Black Shales. 



Many black or blackish blue shales of various horizons show 

 characters which stamp them as sapropelargillites. The best known 

 of these is the Posidonia shale of the West European Upper Lias 

 (Lias € ). This shale, so named from the abundance of the pe- 

 lecypod Posidonia bronni, Voltz, is typically developed in Wiirttem- 

 berg, where especially the locality of Holzmaden near Stuttgart 

 has become famous on account of the wonderful preservation of the 

 great marine saurians found in these shales. Other localities are 

 Whitby and Lyme Regis in England. The wide distribution of this 

 formation, as well as its organic contents, proves it to be of marine 

 origin, though from the nature of the occurrence of the fauna, as 



