NEAR THE WESTERN COAST OF ANGLESEY. 259 



some extent, intermediate between them and the oli vine-diabases *, 

 that it is, in fact, (or has been) an augite- (or hornblende-) olivine 

 rock, rather than an olivine-augite (with or without enstatitef), and 

 that felspar is a common accessory mineral. Consideration, however, 

 of these analyses, and of the structure of the Anglesey rock, both 

 macroscopic and microscopic, fully justifies us in retaining it among 

 the plexites or palseopicrites, if the latter term be preferred. 



It is evident from the above observations that this rock is likely 

 to be commonly met with in Anglesey ; and I have described it with 

 minuteness in the hope that geologists will be on the watch for its 

 occurrence. I have not yet been able to hear of its being found 

 ■in situ in the Welsh area ; but at the end of last year my friend Mr. 

 Teall called my attention to a rock which he had collected at Little 

 Knot, on the east side of Bassenthwaite Lake, which bore, macro- 

 scopically, a considerable resemblance to my specimens. 



This rock is described as a diorite by the late Mr. Clifton Ward, in 

 the Memoirs of the Geological Survey (Lake District), and a figure of 

 its microscopic structure is given. He remarks on the abundance of 

 hornblende, and the " very little felspar," but does not seem to have 

 suspected that olivine had been present, although, from the figure, 

 I have little doubt that such was the case. A slide cut from Mr. 

 Teall's specimen exhibits well-marked crystals of hornblende, both 

 green and brown, with rounded serpentinous interruptions, and the 

 same pale edging as is seen in some of the Welsh specimens ; there 

 is much fibrous actinolite in the body of the slide, and various 

 patches with the peculiar belonites crossing one another, as in some 

 bastite. In many of them are minute rounded specks of granulated 

 aspect, dustier at the edges, and giving with crossed nicols a pale 

 whitish colour. There is a little apatite, a decomposed iron oxide, 

 probably ilmenite, and one or two crystals of felspar, so much 

 decomposed as hardly to be recognizable. The specific gravity of 

 this rock is 2*93. A partial analysis (duplicate), kindly made for 

 Mr. Teall by Mr. E. K. Stock, gives Si0 2 = 46-37 and 46-42, 

 MgO = 15-73 and 15-30 j. These percentages are not in very 

 exact agreement with those in the Welsh specimen (Si 2 = 42-94 

 or 42-79, and Mg = 16-32 or 16*22); but the rock is evi- 

 dently a variable one, and in mineral composition, microscopi- 

 cally and microscopically, there is a very near resemblance. It 



* These have, as a rule, a higher percentage of Si0 2 and still more of A1 2 3 . 

 The analyses of troktoiite, especially of that from Yolpersdorf (see Houghton, 

 Geol. Mag. dec. ii. vol. vi. p. 501), have a closer correspondence with that of 

 this Welsh picrite ; but troktoiite is at once distinguished by its poverty in a 

 pyroxenic or hornblendic mineral. In it also felspar is conspicuous, while in 

 the picrite it is rare. 



t In some of these, i. e. the peridotites (and the serpentines resulting from 

 their alteration), the percentage of magnesia somewhat exceeds that of silica ; 

 this would be the case where olivine was far the most abundant mineral. An 

 increase in the amount of theenstatite would correspond with a rise in the SiO., ; 

 an increase in the augite would be indicated by a marked percentage of CaO. " 



\ 1 have since received, through Mr. Teall, other specimens from Mr. A. 

 Bloomfield, of Keswick. He states that there are seven or eight outcrops of the 

 rock, one of which is quarried. The rock is evidently very variable in character. 



