Ch. XV.] OF THE PRIMARY ROCKS. 345 



ment in trap. In Pembrokeshire, says De la Beche, " besides 

 the above-mentioned varieties of trap, there occurs one which 

 has the appearance of conglomerate. The base is a black 

 and very compact trap, which contains grains of quartz, and 

 seeming fragments of crystallised felspar and of compact fel- 

 spathic cornean. It is probably, however, a pseudo-con- 

 glomerate, or concretional rock, originating in a peculiar and 

 contemporaneous aggregation of its constituent parts." * And 

 of the same nature are the orbicular granite and porphyry of 

 the island of Corsica, and the trap rocks around Edinburgh 

 and in other parts of Scotland; and surely the globular 

 structure of neither can be attributed to mechanical action. 

 So likewise the spheroidal and concentric arrangement of the 

 laminae in the sandstone at Dunbar, which is referred to the 

 influence of ignited trap, but which, however effected, has 

 probably been superinduced since the accumulation of the 

 original incoherent materials of this sandstone ; for we cannot 

 suppose that these spheroids have been formed by attrition ; 

 and such an origin is still less applicable to the concentric 

 globules in the travertin of Trivoli, and in similar tufaceous 

 rocks, which are in a state of constant and progressive de- 

 position from calcareous waters. 



The brecciated appearance of some primary rocks can 

 rarely, if ever, be confounded with that of true fragmentary 

 formations; for one or other of their ingredients will be 

 found assuming such delicate or intricate ramifications as 

 cannot be reconciled with a mechanical origin. It is true 

 that this may not be visible in every specimen, but it cannot 

 fail to be observed when these rocks are examined on a 

 large scale: and farther, the ingredients will commonly be 

 found, here and there, gradually passing into each other by 

 mineral transitions. It is very probable that cases have been 

 observed in which the same characters are possessed by se- 

 condary rocks ; but this would not be any objection ; it would 



* Geol. Trans. (New Series), vol. ii. p. 3. 



