OP LITTLE KNOTT (CUMBERLAND). 517 



probably labradorite. The pyroxenic constituent is much altered 

 and is replaced by secondary microlithic products mostly of a horn- 

 biendic character. Besides the usual decomposed granules of iron 

 peroxide, there is a fair amount of well-marked apatite, which is 

 wanting in the other slides. ]S'o one would imagine that these 

 specimens could have come from one and the same dyke. 



I have to thank Prof. Hughes also for aUowing me to re-examine 

 specimens collected by the late Prof. Sedgwick and Mr. Tawney, 

 now in the "Woodwardian Museum, one of which is noticed briefly 

 in my fu'st paper *. It is now evident to me that in all probability 

 the true picrite collected by Prof . Sedgwick from Penarfynydd (Aber- 

 daron) forms a part of the olivine-diabase described by Mr. Tawney. 

 In all the specimens there is the same characteristic of fair-sized horn- 

 blende crystals interrupted by serpentinous enclosures, so that we may 

 regard this as another outcrop of a similar picrite. This also a^Dpears 

 to exhibit the usual variability in mineral composition. Another 

 series of specimens come from the neighbourhood of Clynnog, the 

 the most characteristic being from a boulder under Gyrn Goeh ; but 

 a similar rock occurs hi situ at Pen-}'-rhiwan (Coll. Tawney) f. 

 These rocks are very similar to the picrites which I have been 

 describing. I doubt whether there has been so much felspar as the 

 authors of the earlier description supposed. 



The variability of the mineral composition of the rocks described 

 above is singular, but perhaps does not indicate quite so great a 

 diversity of chemical composition as at first sight would appear. 

 Looking at the table of analyses given in my last paper, we see that 

 not seldom 1 per cent, of soda is present J. If this were employed 

 in the composition of a felspar like labradorite Si02=5::?-9, 

 Al,O3=30-3, CaO = 12-e3, Na,0 = 4-5, the foUowing constituents 

 would be required to make up the felspar : — 



SiO, = ll-7 

 Al,03= 6-7 



CaO= 2-7 

 ]S'a,0= 1-0 



221 



or more than one fifth of the whole would be felspar. Besides soda 

 there is often a little potash. jSTow as the amount of felspar in 

 all of these picrites which I have examined is nothing like one fifth 

 of the whole mass, it follows that not only the potash must enter 

 into the composition of other minerals {e. g. a mica, often present) 

 but also that some of the soda must be so employed. Further if all 

 the soda in the specimen analyzed by Mr. Phillips went to form felspar, 

 only about 6 parts of alumina would be required ; yet the rock contains 



* Quart. Journ. Geol. Soc. vol. xxxxii. p. 139 : for full description see Gaol. 

 Mag. Dec. 2, vol. vii. p. 208. 



t For description see Geol. Mag. Dec. 2, vol. vii. p. 457. 



:j: In the Anglesey specimen analyzed by Mr. J. A. Phillips there is (mean), 

 NaP^-Qld, K.-f)-l2b. Total of alkalies = 1-04. 



