136 G. H. Kinahan — Laccolitcs. 



The rocks in the more marked laccolites are usually gabbros or 

 allied eurites, that graduate into granitoid, and allied basic el vans ; but 

 sometimes there are also felstones with their allied elvans. Associated 

 with these normal intrusive rocks are others of fragmentary character, 

 like agglomerates and other tuffs. Such mechanically formed rocks 

 are usually supposed to be accompaniments of surface accumula- 

 tions ; a little consideration, however, will show that it is not only 

 possible but even highly probable that they accompany the 

 formation of some laccolites. 



In order that a laccolite may be formed in a particular place, some 

 favourable conditions must exist at that place. During the dis- 

 turbances of the strata, such has taken place in the Cambro-Silurian 

 rocks of Wicklow and Wexford, the horizontal jamming of one or 

 more breadths of rock against each other would make them tend to rise 

 and yield more readily to the pressure of the molten mass injected 

 beneath. In some cases they might rise even independently of this 

 pressure, leaving a vacancy under them, inviting the ingress of the 

 lava ; such hollows might sometimes be enlarged by the gases being 

 forced into them under high pressure, the gases blowing in, out of 

 the passages, loose fragments of the rocks in addition to those 

 carried in on the molten matter ; and all brought into the chamber, 

 either by the force of the gas or by the molten matter, be driven 

 into the cracks or other vacancies, or be lifted up on the surface of 

 the latter. 



The general character of the laccolites under consideration seems 

 to be this — their principal mass or nucleus is composed of intrusive 

 rocks, while on these and filling interstices in the " baiked rocks " 

 are these fragmentary rocks, while " baked rocks " envelope all. 

 Sometimes, however, these fragmentary rocks extend away into the 

 " baked rocks." An explanation for these also may be suggested ; 

 open fissures existed between beds of strata or across them, all of 

 which had to be filled ; into those that terminated either upward or 

 lengthways the fragmentary matter was blown and forced to remain 

 in them, while if the fissure led to another cistern or to the surface, 

 the fragmentary matter would be forced through or carried out of 

 it ; thus we should have dykes of the normal rocks of the laccolite 

 leading from one to another, while on these normal rocks and in 

 dykes or in apparently interbedded masses leading away from them, 

 we should find their fragmentary adjuncts,' 



The fragmentary rocks associated with the gabbros are often 

 highly calcareous. In many cases they are an agglomerate con- 

 taining limestone concretions ; and in some places there are masses 

 of such agglomerates that appear to be independent laccolites, the 

 rocks surrounding them being baked ; in some places it is evident 

 that small masses of such fragmentary rocks must have been pro- 

 truded into their present positions. 



' Gilbert mentions as adjuncts to his laccolites "dykes and sheets"; these, how- 

 ever, are filled with a rock the same as the laccolites, while in Wexford and Wicklow 

 the dykes and sheets often seem to be fragmentary rocks. 



