322 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



of the junction being either covered up by the boulder clay and 

 pleistocene deposits, or where uncovered on the sea-shore, so greatly 

 changed by igneous action as to have lost entirely the distinctive 

 characters of the two series. The best place for tracing the connex- 

 ion seems to be at the edge of the fault at Poolvash, about 300 

 yards westward of the road running from Balladoole to the sea- 

 shore, where some dark beds, which look like the commencement of 

 the lower limestone, are brought up ; but as not more than four or 

 five feet appear, the few fossils contained in them are hardly suffi- 

 cient to establish their identity. There can however be little doubt 

 that a great and almost sudden change took place in the physical 

 condition of the basin in the midst of the period of the deposition 

 of the limestone strata, almost every species of mountain limestone 

 fossils being crowded within a thickness of not more than sixty feet 

 of limestone, and in an area of scarcely a mile across. Even here 

 however we may remark, that the various beds of the series have 

 individually their own more characteristic fossils. Thus I have 

 found Nautilus oxystoma in the lower Poolvash beds alone, and the 

 same species (but of much larger size) in the lowest dark limestone 

 of Ballahot. The beds next above seem characterized by a Natica 

 and by Cyathophyllum basaltiforme. In the next we have Orthis 

 resupinata and Goniatites crenistriay extremely common ; and later 

 still, Nautilus sulcatus. Some of these fossils range more or less 

 through the whole series. The Orthoceratites in general are more 

 common in the middle period, and so also is Terebratula excavata. 

 Somewhat earlier we have Producta anomala, and a little Crustacean, 

 named by De Koninck Cytherina Phillipsiana, pervades both the 

 earlier and middle beds. In the upper portion the larger corals 

 disappear and give place to Fenestella^ while in that part even the 

 Encrinites are not abundant in proportion to other fossils. 



Another sudden change however seems to have taken place in the 

 physical condition of the basin in the midst of the early carboniferous 

 period. A violent convulsion running along an axis from the Stack 

 of Scarlet, in a direction nearly N.W. by W. (see Map, PI. XV.), would 

 appear to have produced an extensive crack or chasm in that direc- 

 tion, accompanied by an eruption of trappean matter and producing 

 a breaking-up of the strata into a series of troughs, causing at the 

 same time a general elevation of the limestone basin. In the troughs 

 thus formed a highly interesting series of deposits then took place. 

 It would seem that from this period there existed for a long time a 

 subaqueous volcano, acting with more or less intensity, at no great 

 distance from this basin, and probably connected also with the before- 

 mentioned crack or chasm. By this means the basin was constantly 

 being filled up with a series of volcanic products, consisting chiefly of 

 volcanic ashes forming a tuff. But a deposition of limestone beds 

 and shales, similar in their general character to the earlier beds of the 

 basin, Avas going on at the same time ; and these deposits became 

 more or less mixed with volcanic strata, thus forming trappacean 

 limestones and shales, the proportion of the respective ingredients 

 depending in great part upon the relative distance of their respective 



