G. H. Kinahan — Laccolites. 



135 



eruptive rocks now exposed had a deep-seated origin ; the molten 

 rock having filled vacancies in the rocks, and never coming to the 

 surface until they were exposed by denudation or by faults. To qiiote 

 our author, "The lava . . . instead of rising through all the 

 beds of the earth's crust, stopped at a lower horizon, insinuated itself 

 between two strata, and opened for itself a chamber by lifting all 

 the superior beds. In this it congealed, forming a massive body of 

 trap." For these masses of eruptive rocks, Gilbert proposes the 

 name laccolite (Gr. lakkos cistern, and lithos stone). In the 

 Cos. Wexford and Wicklow some of the protrusions of eruptive 

 rocks are entitled to this name, the rocks having congealed in cisterns 

 below the surface of the earth ; there are, however, some marked 

 differences between them and the laccolites of the Henry Mountains. 

 The latter were intruded into nearly horizontal strata, the laccolites 

 only consist of one kind of rock, while the adjoining rocks seem to 

 have been very little altered. But the Wexford and Wicklow 

 laccolites, on the other hand, were intruded into highly disturbed 

 strata, they are made up of a variety of rocks, and always the 

 aquo-igneous action due to their intrusion — " baked " or altered. 

 a greater or less thickness of rocks about them. 



Vertical section of a Laccolite in contorted strata. 



Baked rocks. 



+ + 



Molten rocks 



Fragmentary rocks 



of Laccolite. 



The distance to which the "baking" has extended is very variable, 

 on account of the ends, not the planes, of the differently composed 

 beds being in contact with the eruptive rocks ; rocks of diff"erent 

 characters and composition being diff'erently susceptible ; conse- 

 quently, in regard to their composition, some have been more 

 altered in depth and quality, — few apparently being ever changed 

 into gneissoid and granitoid rocks. 



