142 J. W. JUDD ON THE STRUCTURE AND 



fragmentary and scoriaceous materials. The former consist of gabbro 

 (often more or less completely altered to serpentine), dolerites, and 

 basalts ; the latter of cindery materials, mingled with angular frag- 

 ments of various rocks, the whole converted, by the deposition of 

 soluble materials (such as quartz and the carbonates of lime, magnesia, 

 and iron) in their cavities, into solid rocks of peculiar and, at 

 first sight, puzzling character. These great fissures or dykes fre- 

 quently exhibit clear evidence of having been the channels of a 

 succession of volcanic outbursts. In some cases a great mass of 

 fragmentary materials is observed, penetrated by dykes or ducts of 

 consolidated lava ; in others a great vertical mass of lava is found 

 to have been rent asunder, and its fissures are seen to be filled with 

 fragmentary materials. In fact, we are enabled to study in Forfar- 

 shire those deep-seated phenomena which must necessarily be pro- 

 duced when, as is so frequently the case in volcanic districts, great 

 subterranean fissures are opened, giving passage to liquefied rocks 

 and entangled vapours, by the liberation of which, as they reach the 

 surface, cones of cinders and streams of lava are produced at various 

 points along the line of fracture. 



In the second of the districts we have referred to, namely Fife, 

 we may observe another phase of the phenomena of volcanic out- 

 bursts. In consequence of the more mitigated action of denudation 

 upon the surrounding rocks, as the result of the positions they have 

 assumed through subterranean movements, we find, not only the 

 great parallel vertical fissures filled with igneous products, but 

 lateral sheets of lava which, failing to reach the surface, have 

 inserted themselves between the planes of the stratified rocks. The 

 interesting phenomena displayed in such cases I have already noticed 

 in a recent paper. Nowhere can the phenomena presented by these 

 great intrusive sheets of dolerite be better studied than in the Lo- 

 mond Hills of Fife ; and a glance at the beautiful map of the Geo- 

 logical Survey will show how numerous and extensive such sheets 

 of intrusive rock are over the whole district. 



In the third of the districts we have named, that of the Lothians, 

 still higher portions of the volcanic structures of the Carboniferous 

 period have escaped destruction by denudation. Here, indeed, as 

 well as in Southern Fife, we find portions of these products of 

 igneous activity which actually reached the surface, in sheets of 

 lava, piles of agglomerate, and beds of tuff and ash. 



No one studying this series of volcanic fragments in Forfar, Fife, 

 and the Lothians can feel any doubt as to the relationship which 

 they bear to one another. He cannot fail to perceive that, owing to 

 the unequal removal by denudation of their matrices of stratified 

 rocks, different portions of similar contemporaneous volcanoes are 

 exposed to our view. That the dykes and intrusive sheets of Forfar 

 and Fife were surmounted by piles of cinders and currents of lava, 

 is not less certain than that the volcanoes of the Lothians are con- 

 nected with deep-seated dykes and intrusive masses*. 



* That the rocks of Carboniferous age once spread over far wider areas than 

 those which they at present occupy in Scotland has long been a fact familiar to 



