MIDDLE AND UPPEE ORDOVICIAN. 179 



quartzite, the Kittatinny limestone, the Jacksonburg limestone, and the Martins- 

 burg shale. The formations have been described by Ktimmel,"^' ^^^^ from whose 

 .recent account the following notes on the Ordovician are taken. The description 

 of the Cambrian formations has been given in Chapter III (p. 113). 



In northern New Jersey, as elsewhere in the great Appalachian Valley, there is no sharp 

 line of demarcation between the rocks of the Cambrian and those of the Ordovician system. 

 The base of the Ordovician lies somewhere below the top of the Kittatinny limestone, as stated 

 above, but the exact position of this division line can not readily be determined. 



Above the Kittatinny limestone, and separated from it by a break in sedimentation indi- 

 cated by a calcareous basal conglomerate, is a dark-blue or black fossiliferous limestone, corre- 

 lated with the Lowville, Black River, and lower Trenton limestone of the New York section and 

 hitherto classed as "Trenton," some layers of which contain as much as 95 per cent or more of 

 calcium carbonate. Calcareous shales occur interbedded with these limestones and above them 

 to the top of the formation. The sequence of conglomerate, limestone, and shale is a variable 

 one, but, so far as observed, the transition to the overlying formation is always through a series 

 'of calcareous shales which become less and less limy. 



The thickness of the Jacksonburg formation varies from 135 to 300 feet or more. It con- 

 tains an abundant fauna, 98 forms having been described by Weller. At the type locality the 

 lower strata for a thickness of 58 feet carry a Lowville-Black River faiuna, and the higher beds 

 have a lower Trenton fauna. 



The Jacksonburg limestone passes upward through the calcareous shales mentioned above 

 into a great thickness of shale, slate, and sandstone, which has heretofore been known as the 

 "Hudson River slate" but which is now correlated with the Martinsburg shale of West Virginia 

 and takes that name. 



The formation ranges from the finest-grained shale and slate to fine sandstone. The former 

 beds on the whole are black and more abundant in the lower part, whereas the sandstone beds 

 are dark bluish gray, many of them calcareous, and occur more commonly higher in the forma- 

 tion. The fine-grained rocks exceed the gritty beds. 



* * * The whole formation is so crumpled and cleaved that no accurate estimate of its 

 thickness can be made, but it is probably at least 3,000 feet, and it may be more. 



Four species of graptolites of the NormanskiU fauna of New York have been found in the 

 lower portion of the Martinsburg shale, so that the beds in which they occur are equivalent in 

 age to the middle portion of the typical Trenton Hmestone of central New York. A few mUes 

 north of the New Jersey State line Schizocrania and graptoHtes characteristic of the Utica shale 

 of the Mohawk Valley have been found in beds close to the overlying Shawangunk conglomerate. 



Detailed descriptions of the fauna are given by teller .''''^'*''*'*^'' 



K-L 12-13. WYOMING AND NORTHERN COLORADO. 



The Ordovician of this region is a relatively thin deposit of limestone, attaining 

 a maximum thickness of 300 feet, lacking in many sections, and locally underlain 

 by a sandstone which carries remains of fish.^^^ Darton^^'^^ has recently summed 

 up the known facts in a comprehensive manner. 



In the Bighorn Mountains the Ordovician strata constitute the Bighorn dolo- 

 mite, which in the north averages about 300 feet in thickness, including a basal 

 white sandstone, a massive magnesian hmestone, .and about 100 feet of softer 

 thinner-bedded limestone at the top. The strata range in age from Black River 

 at the base to late Richmond at the top and are said to be divided by a hiatus 

 equivalent to later Trenton, Utica, Eden, and Lorraine, of which, however, the 

 evidence is mainly paleontologic. 



