ERUPTIVE AND METAMORPHIC ROCKS. 56 1 



trustworthy lithological evidence of the eruptive character of a rock is the presence 

 of glass, or traces of an original glassy base.' 



In early geological times volcanic action was probably more 

 general and more intense than it is now, but Dr. A. Geikie has 

 remarked that, so far as the records are concerned, we find no 

 evidence of a diminution of volcanic energy in the British area; 

 moreover, the earth's crust has been thickening, and greater force 

 would be required to disrupt it. The earliest evidence of volcanic 

 eruptions is to be found in the later Archaean rocks (Pebidian of Dr. 

 Hicks, Lower Cambrian of Dr. A. Geikie). In the Upper Cambrian 

 period (Ordovician or Lower Silurian) much volcanic material was 

 poured out in North Wales and the Lake District, partly on land 

 and partly beneath the waters, and this material is estimated to be 

 about 5000 feet thick. The eruptive rocks of Cader Idris were 

 formed during the Arenig period, and those of Snowdon in the 

 Bala period ; and the volcanoes of these old times were probably 

 much larger than those of the present day. 



Among the Devonian rocks in Devonshire, and the Lower Old 

 Red Sandstone in Berwickshire and the Cheviot Hills, we find 

 evidence of considerable eruptions ; and again, in the Carbon- 

 iferous period in the 'Toadstone' of Derbyshire there is evidence 

 of volcanic disturbance, which was manifested also at the close 

 of that period. Perhaps the interval between the Carboniferous 

 and Permian periods was marked by the protrusion of the granitic 

 masses of Dartmoor and other tracts in the south-west of England, 

 as well as by some of the basaltic dykes of the north of England ; 

 while the activity of the volcano of Brent Tor may have been 

 proditced during the period of the Lower Culm-measures. Then 

 followed comparatively tranquil times during the deposition of the 

 Lias and Oolites, the Cretaceous rocks, and the earlier portions of 

 the Tertiary Strata. Ultimately the disturbances in the Hampshire 

 and London Basins were produced, perhaps in Miocene times, 

 when there was great volcanic activity in the west of Scotland and 

 north-east of Ireland. 



While we have briefly indicated the contemporaneous outbursts 

 of volcanic material, some of the stratified rocks which furnish no 

 such evidence have in later periods been affected by intrusive 

 rocks, as in the Oolitic district of Yorkshire, where there are dykes 

 of Tertiary age. Nevertheless, while intrusive and interbedded 

 rocks may naturally belong to the same general period, " it cannot 

 always be affirmed that a given mass of intrusive igneous rock, 

 now denuded and exposed at the surface, was ever connected with 

 any superficial manifestation of volcanic action,"^ for these masses 

 often represent incompleted volcanoes. 



Eruptive rocks, as will be presently stated more fully, are divided into two 

 groups, termed the Basic and Acid. To account for tliese broad distinctions 

 Durocher framed the hypothesis that beneath the earth's crust there are two 

 ' magmas ' of heated matter from which erupted material is derived. The lower 



A. Geikie, Text-Book of Geology, ed. 2, p. 524. 

 A. Geikie, Text-Book of Geology, ed. 2, p. 522. 



-.6 



