548 E. B. Taivney — Woodivardian Laboratory Notes. 



avail ourselves of them to the fullest extent ; and I decline to believe 

 that the existing or even the Quaternary distribution of birds and 

 mammals throws much light on the physical geography of the earth 

 when birds were still toothed, and even the primary groups of 

 mammals were scarcely differentiated. 



V. — WooDWAKDiAN Labokatoky Notes, N. Wales Eocks III. 



By E. B. Tawney, M.A., F.G.S. 



{Continued from Vol. VII. 1880, p. 458.) 



AT Penyrhiwan, Clynnog, in the middle of fields, is a protruding 

 boss of dark-coloured rock which has been well broken into, in 

 order apparently to see whether it was available for making " sets " 

 or pitching stones, which form the staple trade of the adjoining 

 district. Its surface is well rounded, as if by glaciation. The boss 

 consists of at least three varieties of rock. Most noticeable is a 

 coarse-grained hornblendic one of handsome appearance, the glistening 

 cleavage planes of the brown hornblende standing out from the re- 

 maining green actinolitic portion. A second variety is a fine-grained 

 greenstone, which is separated from the preceding by a hard definite 

 line, so that it may be taken to be of the nature of a dyke intruded 

 into it. 



A thin slice was taken at the contact so as to show the absolute 

 junction. In the coarse-grained hornblendic rock microscopic ex- 

 amination showed that felspar was almost absent, there was only a 

 very little decomposed pseudomorph that could be put down to this 

 species. The rest consisted either of brown hornblende which 

 passed by all variations into green actinolite of various shades, or 

 decomposition products of the above, chloritic and viridite substances. 

 Apatite is present inclosed in hornblende. Moreover, some secondary 

 brown mica exists pseudomorphing hornblende apparently. The 

 rock to the unassisted eye is very much like one collected by 

 Sedgwick as a loose block under Girn Goch, and a similar rock is 

 much used in the walls. That described however by Prof. Bonney 

 [CI. 15, Sedgwick Coll.], Geological Magazine, Decade II. Vol. VII. 

 p. 457, does not quite agree with ours. Our slice has no augite, 

 while Sedgwick's has abundant octagonal sections of augite showing 

 its cleavage. 



Possibly, however, both have come from the same spot, for there 

 is no other greenstone rock mapped by the Geological Survey near 

 here, nor have we been able to find any. We may perhaps assume 

 that the rock varies in different portions, sometimes augite being 

 present, sometimes not. The present case taken at the junction may 

 be considered therefore a hornblendic-diabase, in which augite hap- 

 pens to be absent, and the felspar at a minimum. 



The dearth of felspar gives the rock an outward resemblance to 

 pikrite, which is also kept up by the dark inclosures which break 

 the reflection of light from the cleavage planes of the hornblende, but 

 we have been unable to determine any olivine pseudomorphs among 

 these. 



