Vol. 52.] LINGULA-FLAGS AND IGNEOUS EOCKS OF DOLGELLY. 519" 



between the two patches of diabase, dip to the south, while the 

 northern diabase-patch dips towards the north. It will be noticed 

 that the northern boundary of the southern patch follows the 

 contour-lines, reducing, however, the sharpness of all the curves ; 

 and this shows that the base of this mass dips towards the south, 

 following, perhaps, as on the western side of the fault, the dip of 

 the beds below. The southern boundary, however, follows the 

 contour-lines very closely, and the diabase near this boundary must 

 therefore be horizontal. 



The form which the mass has assumed is shown in the sections 

 (figs. 1 & 2) on p. 520. The diabase has been intruded, with a 

 tendency to follow the bedding up to a certain height, at which it 

 begins to spread out horizontally. It is clear that the plane along 

 which it has spread must have been a plane of weakness ; but it is 

 not, as in most laccolites, a bedding-plane. Thrust-planes being out 

 of the question in this case, the most feasible explanation is that 

 the plane was one of unconformity; that the Lingula-Flaga and 

 other rocks of this district were overlain unconformably by a later 

 series, that the diabases were subsequently intruded and spread along 

 this unconformity, and that the higher beds have since been removed 

 by denudation. But as we have no trace of the higher beds, this 

 explanation becomes nothing more than a suggestion. 



Mynydd Gader mass.— The form of the Mynydd Gader diabase 

 is similar in some respects ; but here our observations are not yet 

 completed, and we have followed only the northern boundary. If 

 this boundary, as shown upon the map (PI. XXIV.), be examined, 

 it will be noticed that it is not straight but wavy, and although 

 unfortunately the contour-lines are not drawn up to this height, it is 

 clear that the curves are due to the form of the surface. For where 

 a stream crosses the boundary and has cut a little valley, there the 

 boundary recedes towards the south, while between the streams it 

 bulges out towards the north. In the field it is found that the floors 

 of the valleys are formed (where they are deep enough) by the lavas 

 and other stratified beds which underlie the diabase, while the walls 

 are formed of the diabase itself. The surface of separation between 

 the diabase and the other beds slopes down towards the north. 

 The Mynydd Gader diabase, therefore, along its northern boundary, 

 forms a sheet rather than a vertical intrusion. 



At the western extremity of the mass, however, these relations 

 are not preserved. The small elliptical projection at the western 

 end is a hill of diabase almost completely surrounded by hills of 

 nearly equal height which are formed of stratified deposits. The 

 diabase here forms a kind of plug or neck. 



The patch of diabase which lies S.E. of Llyn Gwernan appears to 

 be a sheet dipping towards the north, and doubtless is a continu- 

 ation of the Mynydd Gader mass. 



Concerning the smaller masses but little need be said : all of 

 them show a tendency to run parallel to the strike of the sedimentary 

 deposits, but nevertheless are probably intrusive. The small patches 

 near Brynmawr and Bryn-rhug are very much decomposed. 



Q. J. G. S. No. 207. 2 n 



