GUMMING ON THE ISLE OF MAN. 323 



sources and the intensity of the volcanic action. Occasionally there 

 seem to have been periods of quiescence, allowing larger depositions 

 of limestones unmixed with volcanic tuff. One of the beds so 

 formed is interesting, as having supplied the material for the steps 

 of St. Paul's Cathedral, presented by Bishop Wilson. It is a schist 

 or plate, in some places ten feet thick, in others not so many inches. 

 It is characterized by an abundance of very beautiful and perfect 

 Posidonise, by several cephalopodous shells, not occurring elsewhere 

 in the basin, and in one place by several varieties of Ferns, the 

 nearest approach here to the coal-measures. All the shales abound 

 in iron pyrites; and in one spot we find, a little to the east of a 

 stream running from Balladoole into the sea, and parallel with a 

 trap-dike, several extremely beautiful fossils of this material, con- 

 sisting mostly of Goniatites and Orthoceratites, I have fossils also 

 from the trap-tuff. 



After the deposition of a considerable portion of these trappa- 

 cean limestones and shale beds and a large accumulation of trap- 

 tuff upon them, a second violent convulsion along the same axis as 

 before seems to have taken place, accompanied by a discharge of 

 heated gaseous vapours. The beds formed during the previous erup- 

 tions were broken up, and fragments of trappacean limestone and 

 shale rolled along and mixed in indiscriminate confusion, the con- 

 sequence being a kind of trap-breccia enveloping the broken strata. 

 The trappacean limestone boulders are metamorphic and amygda- 

 loidal, and the shale has become cherty. In some places, where the 

 igneous action may have been more intense, the mass in cooling has 

 assumed a crystalline character, well exhibited at the Stack of Scarlet, 

 where the trap has become columnar basalt, and the adjoining lime- 

 stones have a similar disposition to columnar structure. There 

 seems afterwards to have been a continuation of the tufaceous de- 

 posits, but for how long is altogether uncertain, as there are no car- 

 boniferous or secondary strata which can be determined above 

 them. They are however all intersected by the more modern trap- 

 dikes which stretch in numerous places across the area of the basin. 



The date of the eruption of these dikes may range through the 

 whole interval betwixt the carboniferous and tertiary periods, as they 

 are only covered up by the boulder clay and pleistocene deposits, 

 which occupy the greater portion of tlie area where they occur. On 

 this account the tracing tlie same dike continuously for any distance 

 with certainty is impracticable, and our observations must be made 

 betwixt the high and lower water marks, and the lines filled up by 

 conjecture. Tiie task would not indeed be very difficult did the 

 dikes preserve in general their direction and appearance ; but they 

 are frequently curved, sometimes branching anil thinning out, and 

 ultimately di>a[)pearing, especially where they iiaj)pen to intersect 

 the Old red conglomerate. An extremely interesting section of this 

 kind is laid bare on the western shore of Langness, and is figured 

 in the accompanying woodcut. 



These dikes have more or less altered the intersected beds; and 



