156 H. HICXS ON THE DIMETIAN AND 



were two distinct series of these green bands : — one undoubtedly 

 occurring as intrusive dykes, showing also a columnar structure ; 

 the others running everywhere parallel to one another, and appa- 

 rently interbedded with the quartz rocks. In my last paper (Quart. 

 Journ. Geoi. Soc. vol. xxxiii. p. 229) I kept to this view, and 

 continued to consider these as marked distinctions and as probably 

 indicating a different origin for the two series. It seems now, 

 however, tolerably clear from the examination of specimens of rocks 

 from each series kindly made for me by Prof. Judd, F.R.S., and Mr. 

 T. Davies, F.G.S., that the interbedded masses are microscopically 

 almost identical in character with those which appear as dykes, and 

 hence that both series are probably of igneous origin. Of one of these 

 bands Mr. Davies gives the following description : — " This is a 

 highly altered basic rock, completely chloritized, only the felspar 

 being indicated by its outline. The ground-mass is pervaded with 

 chlorite, while fissures containing the mineral and much calcite 

 traverse it in all directions." 



The following descriptions of specimens from different bands by 

 Prof. Judd are also interesting. 



a. This rock is so similar in character to that which occurs in 

 dykes, that I can find no characters by which to separate it. Only 

 the outlines of the original constituents can be traced, it is true ; but 

 the whole aspect and mode of arrangement is so similar to that of 

 the dolerites, that, unless the geognostic evidence be of the most 

 unmistakable character, it would be well to pause before calling it a 

 metam orphic rock. 



h. More nearly resembles a metamorphic rock than either of the 

 others ; but it presents no characters which would warrant one in 

 asserting that it was not a greatly altered igneous rock. 



c. In spite of the parallel arrangement of the crystals in this rock, 

 I cannot think that it is a metamorphic sedimentary rock. 



The quartziferous breccias described by us in a former paper * 

 are found at several horizons interstratified with the more com- 

 pact quartz rocks. These may possibly be volcanic breccias, but 

 are most probably, I think, compact quartz breccias entirely the 

 result of denudation, and in which the matrix rather readily decom- 

 poses by atmospheric action ; they invariably have a strike of about 

 N/W. to S.E., and hence, across the ridge and parallel to the strike, 

 in the compact quartz rocks under St. David's, and the quartz 

 schists at Porthlisky. One thick band of these brecciated-looking 

 rocks may be seen cropping up at Lower Moor to the south of St. 

 David's ; and another is exposed in the valley still further south on 

 the hillside near the road leading to Porthclais. 



Pebidian. 

 In my former paper I was able only to describe the Pebidian 

 rocks which immediately flanked the Dimetian axis, and had to 



* Quart. Journ. Geol. Soc. vol. xxvii. p. 387. 



