126 
PRE-CAMBRIAN VOLCANOES 
BOOK n 
Xot only are these “ green schists ” traceable all through the Central 
and South - Western Highlands, rocks of similar character, and not ini- 
probaldy on the same horizons, i-eappear in the north-west of Ireland, and 
run thence south-westward as far as tlie Dah'adian rocks extend. If we 
are justified in regarding tliem as metamorphosed tuffs and ashy sediments, 
they mark a widespread and long-continued volcanic period during the time 
when the later half of the Ilalradian series was deposited. 
Besides the extensive development of basic sills which, though probably 
in great part later than the “ green schists,” may belong to the same pro- 
longed period of subterranean activity, numerous acid protrusions are to 
be observed in the Dalradian series of Scotland and Ireland. That these 
masses were erupted at several widely-separated intervals is well shown by 
their relation to the schists among which they occur. Some of the great 
bosses and sills of granite were undoubtedly injected before the metamorphism 
of the schists was completed, for they have shared in the foliation of the 
region. Others have certainly appeared after the metamorphisni was com- 
plete, for they show no trace of having suffered from its effects. Thus some 
of the vast tracts of newer granite in the Grampian chain, which cover 
many square miles of ground, must be among the newest rocks of that area. 
They have recently been found by Mr. G. Barrow, of the Geological Sr^rvey, 
to send veins into the belt of probably Lower Silurian strata which flanks 
the Highland schists. They are thus later than the Arenig period. Not 
impossibly they may be referaljle to the great granite intrusions which formed 
so striking a feature in the history of the Lower Old Bed Sandstone. 
iii. THE GNEISSES AND SCHISTS OF ANGLESEY 
In tlie island of Anglesey an interesting series of schists and quartzites 
presents many points of resemblance to the Ilalradian or younger schists of 
the Llighlands. At present the geologist possesses no means of determining 
whether these Welsh rocks are the equivalents of the Scottish in strati- 
graphical position, but their remarkable similarity justifies a brief allusion 
to them in this place. Much controversy has arisen regarding the geology 
of Anglesey, but into this dispute it is not necessary for my present pur- 
pose to enter.^ I will content myself witli expressing what seems to me, 
after several traverses, to be the geological structure of the ground. 
^ The literature of Anglesey geology is now somewhat voluminous, but I may refer to the 
following as the chief autliorities. The i.slaml is maiiped in Slieet 78 of the Geological Survey 
of England and Wales, and its structure is illustrated in Horizontal Sections, Sheet 40. A full 
account of its various formations and of their relations to each other is given in vol. iii. of the 
Menioi't’s of the Geoloyiettl SuTvey, The Geology of Nortli Wales,” by Sir A. C. Hamsay, 2nd 
edit. 1881. The subject ha.s been discussed by Professor Hughes, Quart. Journ. Geol. N'ac. vols. 
xx.xiv. (1878) p. 137, xxxv. (1879) p. 682, xxxvi. (1880) p. 237, xxxviii. (1882) p. Hi ; Brit. 
Assoc. Rep. (1881) pp. 643, 644 ; Proc. Camh. Phil. Roe. vol. iii. pp. 67, 89, 341 ; by Professor 
Bonuey, Quart. Jouru. Geol. Soc. vol. .xxxv. (1879) pp. 300, 321 ; Geol. Mag. (1880) p. 125 ; 
by Dr. H. Hicks, Quart. Journ. Geol. Soc. vols. xx.xiv. (1878) j). 147, xxxv. (1879) pi. 295 ; Geol. 
May. (1879) pp. 433, 528 (1893) p. 548 ; by Dr. C. Callaway, Quart. Jourti. Geol. Soc. vols. 
xxxvii. (1881) Ji. 210, xl. (1884) p. 567 ; and by the Rev. .1. K. Blake, Quart. Jo-urn. Geol. Soc. 
