GKOLOGY OK OEKBYSH1KK 2*9 



M. Agassiz to have been of a very remarkable kind, having many 

 of the characters of a saurian reptile, whence they have been called 

 Sauroid fishes. Some of the teeth found near Chesterfield are coni- 

 cal and striated, more like those of an Ichthyosaurus than the flat 

 teeth common to fish : others, however, are palatal teeth, similar to 

 those found in the chalk, but differing from them in form and mark- 

 ings. 



The vegetable remains of the coal measures are far too numerous 

 and form too complicated a subject to enter upon in this sketch. I 

 must, therefore, refer my readers to Lindley and Hutton's Fossil 

 Flora for a general description and figures of coal plants, and con- 

 tent myself with saying that the vegetable remains of the Derby- 

 shire coal measures do not differ from those of other coal fields. 

 Many parts of the district seem very barren in fossils; in others 

 these are abundant. The genus Calamiles is common, Lepidoden- 

 dron not unfrequent, but Sligmaria and Sigillaria seem more rare 

 than usual, neither is there that abundance of beautiful fern leaves 

 which may be seen in some places, especially in the Ashby coal 

 field, as described in Mammatt's Geological Fads. The truth is, 

 however, that sufficient attention has not yet been bestowed upon 

 the district ; and there is no doubt it would amply reward the perse- 

 verance of local collectors, whose labours we hope will ere long be 

 devoted to it. 



8. — The Limestone Shale 



Is the next group in order below the millstone grit. This is for the 

 most part an immense mass of shale : in its upper portions, how- 

 ever, it frequently contains beds of gritstone, and in its lower, beds 

 of limestone, showing a passage by insensible gradations into the 

 rocks above and below it. This shale does not differ in character or 

 appearance from many of the shales of the coal measures; it is, 

 however, more universally black, and it stands exposure to the wea- 

 ther for a much longer time than the generality of the coal shales. 

 It splits commonly into remarkably thin laminae, contains many 

 ironstone balls, and sulphur may frequently be seen in its crevices 

 or lying in it in streaks and patches. The water which runs 

 through it is generally ferruginous. Its thickness appears to be 

 variable, the average being probably five or six hundred feet. In 

 the southern part of its range, however, it occupies a wide surface, 

 and here it probably becomes thicker. It contains here, in its low- 

 er portions, many beds of limestone, which may be seen to advan- 



