272 J. R. Dakyns — Geology of Snoinlon. 



of intrusive rock parted by a band of felsiLe about seventy yard* 

 wide. Tlie upper of these is a massive unsbeared diabase of the 

 ordinai'y type. The lower is exactly like the Cwm Llan rock, with 

 which it is doubtless connected underground. Both of these masses 

 (viz., the Cwm Llan rock and the lower of the basic rocks near Llyn 

 Nadroedd) are highly complex, including a considerable variety of 

 material, of the mutual relations of which, as well as of tbeir 

 relation to the surrounding rocks, I hope to be able some day to give 

 a detailed account.^ The bulk of the rock is a dark-green schist, 

 with a highly developed parallel structure and a fine silk}' lustre on 

 the foliation planes. Under the microscope the rock is seen to bo 

 completely reconstructed and recrystallized, to be a finely felted or 

 schistose aggregate, in fact, a perfect crystalline schist. Chlorite, 

 indeed, forms the bulk of the rock, with granules of epidote, 

 magnetite, and leucoxene; but some specimens are largely composed 

 of minute needles with a much higher double refraction than 

 chlorite, which can hardly be anything but a pale-green hornblende, 

 produced apparently at the expense of the chlorite, and the result 

 therefore of a more advanced stage of metamorphism. A curious 

 feature is the presence in some of these schists of a large number 

 of small concretions (up to the size of large peas). These are 

 composed of a quartz mosaic, with generally a border of radially 

 arranged needles of a green mineral, chiefly chlorite, though horn- 

 blende is also present. Some are perfect little radial spherulites. 

 Many of these concretions can be seen to cut sharply across the 

 foliations, and no sign of strain has been observed in their minerals, 

 so that it is clear that they are not amygdules, but that they have 

 grown since the last deforming stresses to which the rock was 

 subjected. 



The character of these rocks is so different from that of any other 

 of the Snowdonian rocks that the thought crossed my mind whether 

 they might not be a boss of Archaean rocks sticking up in the valley 

 bottom; but this idea, to say nothing of the structural difficulties, 

 was negatived by finding that the metamorphic rock envelojied 

 masses of grit, and was therefore intrusive. 



12. Before closing this summary of my work, I may as well 

 mention that there is a mass of basic intrusive rock, not shown on 

 the Geological Survey Map, amid the rocks of the calcareous series 

 S. by E. from Cwm Dyli waterfall and nearly opposite Gvvastad 

 Agnes, which I have not yet mapped ; and that I have found and 

 mapped a boss of diabase of the Siabod type, not shown on the 

 Survey Map, from 450 to 700 yards N.N.W. of Pen-y-gwryd. 



I find, too, that the upper part of the felstones of Glyder Fach on 

 the southern slope of the hill consist of breccias which pass up 

 gradually into grits. 



13, In conclusion I will briefly state the chief things discovered 

 by me in the Snowdon area : these are, first, the fact that much of 



^ The following mineralogical description has heen communicated to me by 

 Mr. E, Greenly. 



