

Vol. 52.] LINGULA-FLAGS AND IGNEOUS KOCKS OF DOLGELLY. 521 



VI. Age of the Intrusive Rocks and of the Faults. 



The three chief faults shown upon the map run nearly, but not 

 quite, parallel to one another, but they are not of precisely the 

 same age. The Nant Ceunant fault is clearly posterior in date 

 to the intrusion of the diabase ; and, as we have seen, this is 

 probably true of the Derwas fault also. The Dolgelly fault, on the 

 other hand, has little influence upon the intrusive mass, although 

 its effect upon the stratified deposits is very marked. There is, 

 however, evidence that a small amount of movement took place 

 along the plane of this fault after the intrusion of the diabase. 

 The sections drawn on the two sides of the fault (figs. 1 & 2) show 

 that the diabase spreads at different levels. North-west of the 

 fault the lateral expansion of the diabase takes place at a height 

 of 400 feet above the sea, and south-east of the fault at a height 

 of 450 feet. This seems to indicate a downthrow of 50 feet on the 

 north-western side since the intrusion of the diabase ; while the 

 downthrow of the stratified deposits (which took place before the 

 intrusion) is on the south-eastern side of the fault. 



The actual age of the Dolgelly fault is not itself determinable, 

 but it runs parallel with the great Bala fault which, on the other 

 side of Cader Idris, lies in the valley of Tal-y-llyn. The Bala 

 fault is Upper Carboniferous or post-Carboniferous in date, for in 

 its extension to the north-east it frequently faults the Carboniferous 

 Limestone and Millstone Grit. Some geologists may be disposed to 

 infer that the Dolgelly fault is of the same age as the Bala fault, 

 and that the diabase is therefore post-Carboniferous in date. But 

 this must still remain open to doubt. 



If the plane along which the diabase near Dolgelly has spread in 

 laccolitic fashion is one of unconformity, the unconformity must be 

 newer than the Dolgelly fault, for the lateral expansion of the 

 diabase takes place at nearly the same level on the two sides of 

 the fault — if anything even at a higher level on the downthrow 

 side. If, therefore, the plane is one of unconformity, either the 

 Dolgelly fault must be pre-Carboniferous, or the unconformity must 

 be a newer one than that at the base of the Carboniferous. 



VII. Conclusions. 



In conclusion we believe we have shown that: (1) outflows of 

 andesitic lava occurred in this region as early as the period of the 

 Upper Lingula-Fl&gs ; (2) at some subsequent date intrusions of 

 diabase took place, which show a tendency to spread out more or less 

 horizontally along a certain plane, perhaps a plane of unconformity ; 

 (3) the diabase in some cases, as it flowed along, dragged with it great 

 masses of the underlying rock : (4) faulting took place both before 

 and after the intrusion of the diabase ; and in one case there seems 

 to have been movement along the plane of the fault in one direction 

 before the intrusion, and in the opposite direction after the 

 intrusion. 



2n2 



