215 



hundred leet and also as vertical, or nearly vertical, dykes. Professor 

 RAMSAY says : " It is not to be supposed that the greenstones are in this 

 reo-ion all precisely of one date, but for many reasons it may be considered 

 certain that the majority of those that lie more or less between the beds 

 were injected in Lower Silurian [Ordovician] times, in which case they 

 were of prior date to the disturbances of the rocks, and consequently to 

 the development of the cleavage of the country. This* however, is not 

 invariable, for it will be shown that in Caernarvonshire some of the common 

 vertical dykes include fragments of cleaved slate. These, I believe, were 

 injected at a much later date." (1) 



The number of separate exposures of " greenstone " in Merionethshire 

 and Caernarvonshire must be reckoned by hundreds, if not thousands. Up 

 to the present time the rocks have not received much attention at the 

 hands of modern petrographers. Mr. TAWNEY has given us an account of 

 some of those occurring in the Lleyn Peninsula of Caernarvonshire and his 

 papers < 2) contain almost all that is known as to the precise composition of 

 the augitic greenstones of North Wales. Many specimens and sections of 

 the rocks from various localities exist in public and private collections. 

 The following remarks are based on the examination of some of these and 

 on Mr. TAWNEY'S published observations. The older observers regarded 

 the " greenstones " as being largely composed of hornblende. This is now 

 known not to be the case, at least to anything like the extent formerly 

 supposed. Hornblendic greenstones (diorites) occur in the Lleyn 

 Peninsula and Anglesea, and probably also in Merionethshire, but they 

 are certainly subordinate to the augitic greenstones (diabases). 



If a collection of the augitic greenstones of North Wales be placed 

 side by side with a collection of Carboniferous or Tertiary dolerites, little 

 difficulty will be found in distinguishing them. Here and there individual 

 specimens may be seen to be identical, but taken as a whole each collec- 

 tion will have its distinctive characters. Thus chlorite is an almost 

 constant feature in the greenstones ; hence their name. Olivine as a rule is 

 unrepresented either in the fresh or altered condition. The plagioclase is 

 generally turbid and sometimes almost opaque in consequence of alteration. 

 The ragged plates of ilmenite are generally more or less changed to 

 leucoxene. It must indeed be confessed that none of these characters 

 taken by itself is distinctive of the Lower Palaeozoic greenstones, and yet 

 taken together they give a general aspect to any extensive collection of 

 these rocks by which it may be distinguished from a similarly extensive 

 collection of British Tertiary or Carboniferous dolerites. 



There is one special type of rock which is very widely distributed in 

 Merionethshire and Caernarvonshire. It is a msdium grained, dark 



(1) A specimen in the ALLPORT collection in the British Museum from the neighbourhood 

 of the South Stack Lighthouse, Holyhead, Anglesea, is a wonderfully fresh ophitic olivine- 

 dolerite. In its composition and state of preservation it differs so markedly from the 

 "greenstones," and approaches so closely many of the Carboniferous and Tertiary dolerites 

 that one is inclined to regard it as a rock of much later date. 



(2) Woodwardian Laboratory Notes, G.M., 1880, pp. 207 and 452 ; Ibid., 1882, p. 548 

 Ibid., 1883, pp. 17 and 65. 



