appearing to be the stalagmite formed on the bottom of the cavity, and proceeding from below upwards in successive additions, in a way similar to that which may be observed in the complicated chalcedonies of Faroe. The peculiarity observable in the fourth example can be explained only by a similar supposition.
Such are the circumstances of chief importance which occur in the trap nodules, and which, although by no means limited to this place, appear deserving of notice, on account of the intimate relation they bear to any general theory. It is incumbent on that hypothesis which explains one of these difficulties, to explain the others also, or at least to require nothing which shall exclude the means of explaining them. It is almost needless to say that I here allude to the different explanations which the aqueous and the igneous theories of the origin of trap have given of the imbedded nodules. Each hypothesis has its difficulties when it refuses to yield ground to its antagonist; yet, is each perhaps incapable of exclusively and clearly explaining the appearances in question. The stalactitical forms which line the hollow cavities, can perhaps have resulted only from watery infiltration, yet the existence of the previous cavities can scarcely be accounted for by that hypothesis which considers trap as a deposit crystallized from an aqueous solution of earths.
Different substances are found occupying veins as well as nodules in this rock; among these, calcareous spar and quartz are the most common. Sulphate of barytes occurs more rarely, as do chert and agate, the latter of which often exhibits the same zoned appearance in the veins which it does in the nodules.
The great abundance of chlorite in this trap might have rendered it probable that the rare mineral heliotrope was an inmate of it:
- Pl. 10. N° 4.