part 3] LOWER PALEOZOIC or abthog-dolgelley. 



287 



normal acid granophyre (fig. 3). The origin of the intrusion- 

 breccias and the local movements is to be looked for in the 

 existence precisely at this point of thin strips and lenticles of ash 

 enclosed within the lower part of the intrusive mass. The strips 

 are hardly big enough to be shown on the map (PI. XX). Neither 

 were they big enough to interfere with the process of differentiation 

 in place, so that the basic marginal rocks occur in their usual 



Fig. 3. — Intrusion-breccia , Pared-y-Cefn Sir. 



a. k. w. del. 

 [The basic marginal rock is dotted, the granophyre is left unshaded.] 



position. But, as the magma cooled and contracted, the lenticles. 

 appear to have undergone a settling-down, the result of which is. 

 apparent in the reversed dip of some of the smaller masses. The 

 settlement of solid material opened new channels enabling the still 

 molten acid material of the intrusion to invade the already con- 

 solidated differentiation -product at the margin. For the greater 



