Geology of Antigua. 83 
from the partial examinations already made, that they are due, 
not to a single cause, but to a combination or rather a diversity of 
causes. For example, some of the finest specimens of jasper are 
found in trap veins, and in the oie trap rocks. _ There 
can be no doubt, therefore, that these are to be ascribed to igne- 
ous agency, converting an aqueous rock into this beautiful sub- 
stance.’ Lyell, De La Béche, and other authors, have detailed 
similar facts occurring in other parts of the world. But in regard 
to the chert deposits, and the immense quantities of petrified 
wood connected with them, I think we must look for the agency 
of some other cause. The circumstance that those beds contain 
shells, either marine or fresh water, or both, is indubitable evi-. 
dence, that they are an aqueous deposit. But whether they were 
originally deposited in their present form, or whether they are al- 
tered rocks, is a question about which there may perhaps be some 
difference of opiriion. It is perfectly obvious, that since their 
formation, they have been subjected to the action of an internal 
force, which has thrown them up and broken them in pieces, and 
perhaps in some degree changed their constitution. The island, 
also, in the trap formation and in the contiguous altered rocks. 
affords the most ample evidence of comparatively recent igneous 
1 
- action on a broad scale. The position of the strata, also, being 
conformable with those’of the clay formation and not separated 
by any definite lines, might be considered as favoring the suppo- 
sition, that they both belonged originally to the same class of 
rocks. ‘Though I know of no example on so large a scale, where 
rocks of this description can clearly be traced to such-an origin, 
yet cases of a more moderate extent are not unfrequent. And if 
we admit, with Lyell, that all the earlier slates aré merely meta- 
morphic _ rocks, oteeae from. one and other fragmentary 
their it semi-crystalline forms by internal heat, 
we seem to have an  atktiowledged eause adequate to the effect. 
But, however sublime and interesting such a conception’ may be, 
we are not perhaps yet prepared to admit it among the sober 
truths of geology. But independently of this objection, there are 
pecs circurnastaaces, which seem to refer the beds in question 
have already remarked that the shells im- 
edited in them show w, that they were originally deposited from 
water; and the fact that these shells are peculiar to the chert— 
that is, are not found in strata of the clay formation—seems to be 
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