PART II. CHAPTER XXIW. 



279 



Trap coeval with Coal and Old Red Sandstone. 



many angular fragments of trap porphyry, some of them one or 

 two tons in weight, intermingled with pebbles of other rocks. 

 These angular fragments were probably thrown out from vol- 

 canic vents, and fell upon sedimentary matter, then in the course 

 of deposition.* 



Carboniferous period, — Two classes of contemporaneous trap 

 rocks have been ascertained by Dr. Fleming to occur in the coal- 

 field of the Forth in Scotland. The newest of these, connected 

 with the higher series of coal-measures, is well exhibited along 

 the shores of the Forth, in Fifeshire, where they consist of basalt 

 with olivine, amygdaloid, greenstone, wacke, and tuff. They 

 appear to have been erupted while the sedimentary strata were 

 in a horizontal position, and to have suffered the same disloca- 

 tions which those strata have subsequently undergone. In the 

 volcanic tuffs of this age are found not only fragments of lime- 

 stone, shale, flinty slate, and sandstone, but also pieces of coal. 



The other or older class of carboniferous traps are traced 

 along the south margin of Stratheden, and constitute a ridge 

 parallel with the Ochils, and extending from Stirling to near St. 

 Andrew's. They consist almost exclusively of greenstone, be- 

 coming, in a few instances, earthy and amygdaloidal. They 

 are regularly interstratified with the sandstone, shale, and iron- 

 stone of the lower coal-measures, and, on the East Lomond, 

 with Mountain limestone.f 



Trap of the Old Red sandstone period. — -By referring to the 

 section explanatory of the structure of Forfarshire, already given 

 (p. 65.), the reader will perceive that the beds of conglomerate, 

 No. 3., occur in the middle of the old red sandstone system, 1, 

 2, 3., 4. The pebbles in these conglomerates are sometimes 

 composed of granitic and quartz rocks, sometimes exclusively 

 of different varieties of trap, which, although purposely omitted 

 in the above section, are oflen found, either intruding themselves 

 in amorphous masses and dikes into the older fossiliferous tile- 

 stones. No. 4., or alternating with them in conformable beds. 

 All the different divisions of the red sandstone, 1, 2, 3, 4, are 

 .occasionally intersected by dikes, but they are very rare in Nos. 

 1. and 2., the upper members of the group consisting of red 

 shale and red sandstone. These phenomena, which occur at the 

 foot of the Grampians, are repeated in the Sidlaw Hills ; and it 

 appears that in this part of Scotland volcanic eruptions were 

 most frequent in the earlier part of the old red sandstone period. 



The trap rocks alluded to consist chiefly of felspathic porphy- 



*De la Beche, Geol. Proceedings, No. 41. p. 196. 



t P'leming MS. Part of this tract I have myself examined with Dr. F, 



