66 DESCRIPTION OF THE STRATA. 



blueish grey. Where there are most shells, the colour is usually 

 darkest, the substance of the shells, which is for the most part sparry 

 and highly crystalline, having a watery or flinty aspect. The ova, or 

 small globules, often look whiter than the matrix or cement in which 

 they are imbedded; but this may be owing to the greater smoothness 

 of their surfaces, the cement having a more dull and marly appearance. 



The oolite bed varies in thickness, from a few feet to fifty or sixty 

 feet; and at Pickering and some other places its extreme thickness 

 rather exceeds sixty. It is regularly stratified, having seams parallel 

 to the general bed; which seams, as in the chalk, are most numerous 

 in the upper part of the bed, the lower part being generally more com- 

 pact and massive. These seams are crossed by a number of vei'tical 

 fissures, which often descend into the beds below the oolite. The 

 seams and fissures abound with marl or decomposed lime ; and in the 

 vertical fissures, we find vast quantities oi calc-spaVy in a great variety 

 of forms. Some of the forms which it assumes are very beautiful, 

 especially where the fissures are not filled up, but merely lined with 

 it; in which case we often meet Avith the fine pyramidal crystals 

 called dog-tooth spar. 



Among the substances occurring in the oolite, it is necessary to 

 ^nention some singular nodules, that are found in the quarries at 

 Thornton, and other places. They are highly siliceous, yet not with- 

 out some mixture of calcareous matter, particularly on the outside,' 

 where ova, or globules, like those of the oolite in which they are im- 

 bedded, occasionally appear. The nodules are usually of a pyriform 

 or oblong shape, like some of the madrepores found in the chalk, and 

 part of them are real madrepores, but in others no distinct organiza- 

 tion can he perceived. Their chief ingredient may be called chert or 

 hornstone. This forms the exterior part, while in the interior is a kind 

 of nucleus of chalcedony with a mixture of agate. The chert is of va- 

 rious colours, being in some specimens light grey, and in others of a 



