442 Scientific Geology. 



The columnar structure of the trap rocks has never yet received a 

 very satisfactory explanation. From the fact that clay when drying 

 divides into columnar masses, it is natural to enquire whether the col- 

 umns of trap had not a similar origin. There is, however, one insuper- 

 able difficulty in such an hypothesis. The desiccation of the clay 

 always causes it to shrink, so as to leave interstices between the colum- 

 nar masses. But no such shrinking has taken place among the prisms 

 of trap. There are no spaces between them : their sides actually 

 touch. Perhaps in the present state of our knowledge we cannot 

 come nearer the truth, than to consider this columnar structure as one 

 of the forms of a concretionary structure. Yet this solution affords 

 no great satisfaction to the mind, so long as we are in doubt as to 

 the nature of a concretionary structure. 



We have seen that greenstone is sometimes divided by parallel 

 planes into masses exceedingly resembling strata. If they are not 

 genuine strata, what is their orgin ? 



In all the unstratified rocks, small spherical or ovoid concretions 

 occur, composed of concentric layers of greater or less thickness. 

 These concretions vary exceedingly in size, and are sometimes not 

 more than an inch in diameter. Nor do I know of any principles in 

 geology or chemistry that can fix any limits to their size. Suppose 

 now a concretion to have been produced of mountainous bulk. It is 

 obvious that if only a small portion of its surface be laid bare, and 

 only a few of its envelopes penetrated, these layers may have exactly 

 the appearance of strata ; because their curvature is so slight for a 

 small extent as not to be perceptible. In this way should I account 

 for the laminar disposition of the trap at Nahant : and I shall apply 

 the same explanation to some examples still more striking in the sie- 

 nite and granite of the State. In one of these cases I think I have 

 satisfactorily traced out a concretion of mountainous bulk, and thus 

 given a confirmation to the theory I am advocating. 



16. PORPHYRY. 



Porphyry has been for so many centuries used as an ornamental 

 stone, and the term has been so often employed by lapidaries and au- 

 thors who were ignorant of geology, that its popular acceptation is 

 quite too loose for scientific description. Nor are geologists exactly 

 agreed as to its meaning. " The term porphyry," says Macculloch, 

 " is applied to a rock in which crystals of common feldspar are im- 



