NEIGHBOURHOOD OF S\RN, CA-ERXAEVONSHIRE. 459 



presents a stratified arrangement of varying lithological types. 

 Judging by the outcrop and observed dip, the Penarfynydd laccolite 

 must have a diameter of not less than three quarters of a mile 

 and a thickness of more than a thousand feet. These dimensions 

 are exceeded by many of the trachytic laccolites described by 

 Gilbert. 



On Mynydd Penarfynydd the beds which formerly arched over the 

 roof of this great " stone-cistern " have been entirely removed by 

 denudation. There are also complications arising from the effects of 

 the subsequent earth-movements by which the rocks of North Wales 

 were disturbed from their horizontal position. The injection of 

 the laccolite between Upper Arenig strata, doubtless a process 

 accomplished by many successive influxes of magma of varying 

 composition, clearly took place when the strata were horizontal ; 

 and the whole mass was afterwards tilted over into an inclined posi- 

 tion without other disturbance. Under the laccolite the shales have 

 been protected from the lateral pressure by the stubborn resistance 

 of the overlying eruptive mass, and present accordingly a uniform 

 inclination. To the west of Penarfynydd, on the other hand, the 

 strata are much disturbed, with conflicting dips at high angles, the 

 rocks here having been crushed by the thrust from the north-west 

 against the unyielding igneous mass. 



The Penarfynydd laccolite is the only one which can be clearly 

 made out in the field. It is, however, a reasonable conjecture that 

 the mass of hornblende-diabase constituting Mynydd-y-graig is 

 another and larger laccolite, injected at a later time and on a slightly 

 higher horizon, and modified in form at its south-western edge by 

 the mass of Mj^nydd Penarfynydd. The field-relations of the rocks 

 about E-hiw are too much obscured by surface-deposits to admit of 

 any definite conclusions. Though the hornblende-diabase never 

 exhibits the strikingly stratiform appearance of the hornblende- 

 pi crite, there seems to be a certain constant difference between 

 different parts of the large masses. The rock which extends from 

 Careg-llefain along the ridge of Mynydd-y-graig is readily dis- 

 tinguishable from that exposed on the south-eastern slopes. The 

 hornblende-diabase north of Ehiw resembles that of the base of the 

 Penarfynydd laccolite, while the rock of Plas E,hiw and Treheli is 

 of the Careg-llefain type *. 



The date of the picrite and hornblende-diabase intrusions is a 

 matter for inference only. If we suppose, with Sir A. Samsay, that 

 the disturbance of the strata was effected in pre -Llandovery times, 

 these eruptive masses, which share that disturbance, must be referred 

 to some part of the Bala age. Assuming this, the absence from the 

 district of any contemporaneous lavas of like composition is best 

 explained by supposing that the whole of the hornblende-diabases 

 were injected in the form of laccolites. 



* Specimens from Treheli and from Mynydd-j-graig have been described by 

 Mr. Tawney. 



