134 DESCRIPTION OF THE STRATA. 



calcareous matter. The cones are variously aggregated ; the larger 

 containing several concentric cones within them, and one cluster often 

 encroaching on another, or reclining on the side of another, so as 

 almost to make their apices meet. When the stone is broken, the 

 cones are very discernible in the fracture, and may often be takett 

 out singly, or in clusters. They are transversely marked with undu- 

 lating wrinkled lines, which perhaps point out the margins of the 

 successive lamellcB, or coatings, of which tlie cones are composed. 

 This stone has the blueish colour and soft feel of the alum schist. It 

 is occasionally found in patches, not attached to any central block. 



The jet that occurs in this bed, of the finest quality, and often in 

 considerable quantity, will fall to be described in the Second Part of 

 this Work ; for it belongs to the list of organic remains, being obviously 

 a particular modification of petrified wood. 



Having given a description of the main bed of the alum rock 

 (No. 6 of the series at Boulby), let us proceed to a lower part of the 

 rock (No. 7), which we have stated to consist of imperfect seams, 



OR FLAT NODULES, OF HARD BI^UE LIMESTONE, MIXED WITH, OE 

 IMBEDDED IN, ALUM SHALE. 



This part of the strata (marked I) is not, strictly speaking, a new 

 bed, but a continuation of the same bed somewhat modified ; for there 

 is no precise line of demarkation between this part of the strata and the 

 main bed of schistus. When the shale has descended about 180 or 

 200 feet, the imbedded nodules or blocks become larger and more 

 abundant, sometimes running in the shale in lengthened rows, approach- 

 ing to seams. Like the courses of flint in the chalk, they do not form 

 any continued stratum, but the flat masses are so numerous and broad, 

 that they may be traced in a kind of imperfect seams through a consid- 

 erable space. The shale in which these courses of flat masses are 

 imbedded, is similar to that of the main bed, but of a harder quality. 



Among the imbedded masses of this part of the rock, we find 

 the same variety as in the upper part of the schist. Some of them 



