112 Rev. W. D. Conybeave on the Phenomena of Geology 



the different varieties of trap pass into each other by such 

 insensible gradations, that much doubt must often hang over 

 the subject. 



In the so called primitive epoch, we find a very large pro- 

 portion, universally distributed, of rocks which have been re- 

 ferred to an igneous origin by evidence which has produced 

 an increasing effect on the minds of geologists, the more at- 

 tentively it has been examined and weighed. I speak prin- 

 cipally of the granitic rocks ; but I think we must refer most 

 of the'felspathic and amphibolic series to this class. From their 

 general relations, we may probably consider most of these 

 rocks as having originated antecedently to the transition 

 period; but I am far from supposing that any of them are 

 necessarily confined to this age; on the contrary, I believe 

 many granitic rocks, those especially passing into sienite, to 

 belong generally to the transition period, and some to even 

 younger epochs*. 



The transition period likewise embraces a very large pro- 

 portion of rocks, such as greenstones, sienites, &c., apparently 

 of igneous origin ; though here again the same difficulty as 

 to limiting their exact age presents itself: for instance, in 

 Pembrokeshire the graywacke is extensively associated with 

 greenstone, which latter, when it approaches the superjacent 

 carboniferous limestone, breaks through it, thus demonstrat- 

 ing its more recent origin. Yet when we take a general view 

 of the primitive and transition districts, and compare the pro- 

 portion of the rocks generally esteemed of igneous origin, 

 which universally occur associated with them, occupying ge- 

 nerally nearly one-fifth of these districts, with the much smaller 

 proportion of the igneous rocks in the more recent formations, 

 and their frequent absence in the latter case, we cannot, 

 I think, resist the conclusion, that the causes which have pro- 

 duced them were in much more active operation in the earlier 

 period. 



The carboniferous series still embrace many trap rocks, 

 though in a far less proportion, and much more limited to 

 particular localities. Thus they are very abundant in the Scotch 

 carboniferous tract ; less so, but still far from scarce, in that 

 of Northumberland and Durham ; that of Derbyshire scarcely 

 exhibits them, except in the toadstones alternating with the 

 limestone. We find them in overlying masses at Cleehill, and 

 in Staffordshire at Dudley; but the great coal basins of our 

 south-western counties, Somerset and South Wales, scarcely 



* Boue refers the granite of Zinwald in Bohemia to the transition period, 

 and that of Baveno even to the carboniferous. Near Predazzo there is an 

 upright mass of granitic porphyry, said to be younger than the lias. 



present 



