MONIAN SYSTEM OF EOCKS. 471 



tongue of calcareous rock terminating like an intrusive rock amongst 

 the schists. Hence to Porth-y-defaid the chloritic schists con- 

 tinue, with only minor disturbances, and mostly with beautifully 

 ruled horizontal lines. Near that place there is a kind of pustule of 

 hard dolomite and quartz, with specks of copper-green, which runs 

 across the bedding, and seems to be produced by an infiltration of 

 the mineral which has forced the lamina3 asunder. 



All these phenomena, which otherwise might not have required so 

 much detail, prove how inextricably the chloritic schists are con- 

 nected with the types of rock, and even with their accidents, which 

 we shall meet with further north on the other side of Porth-y-defaid. 

 The synclinal of ashy rocks may be also traced inland by the Holland 

 Arms Inn, Llanfachreth, and at the crossing of the Alaw by the 

 high road from Yalley, after which it seems to die away. 



At Porth-y-defaid there is undoubtedly a disturbance and a fault, 

 as stated by Dr. Callaway, and after this we do not meet, for some 

 time at least, with anything even remotely resembling the chloritic 

 schists. At first the rock is soft and tufaceous ; then harder green 

 ashy rocks appear, and are succeeded as we pass into the bay by the 

 typical marbled slate. By the occurrence of this rock we are able 

 to trace the group inland, in a southern direction near Plas-y-glyn, 

 as far as the Llanfwrog smithy. Hence this fault is apparently 

 lost inland, and to the east there is no trace of it in the line of 

 junction, which becomes less and less marked, following nearly the 

 course of the northern branch of the river Alaw. Thus at Pont 

 Scyphydd, on the eastern side, there is chloritic schist with a few 

 sporadic chips ; on the western, at Caerdeon, the rock is unfoliated, 

 and contains many chips, while near at hand is a knob and reef of 

 quartz, characteristic of the upper portion of the series. These 

 observations do away with the importance of the Porth-y-defaid 

 fault, and show that though we can distinguish an upper more ashy 

 portion from a lower more schistose, one passes into the other after 

 the manner of parts of a single series, and is not separated from it, 

 as members of two different groups would be. 



The Nokth-westeen Aeea. — On the northern side of the line 

 just defined the whole of the rocks appear to have a partly volcanic 

 origin, and thus to possess characteristic irregularity of development. 

 Only at Camawg, one mileE.S.E. of Ehydwen, have I seen any rocks 

 resembling the foliated rocks of the south by the existence in them 

 of continuous parallel lines. This small patch may serve to show 

 from the other side the real continuity of the whole series. As 

 explained before, we are only likely to meet the more indurated 

 masses in the projecting knobs which still characterize the inland 

 country, but are now more rare. These show a new type of rock. 

 The rocks are dark in hue, with a sort of indigo-purple tinge, and 

 are somewhat slaty in aspect ; but there is no lamination or bedding, 

 the whole is a solid mass in which the only separating surfaces are 

 of subsequent origin, which may be wanting altogether or divide 

 the rock into concave lenticles, like thickened watch-glasses. These 



