THE MOST ANCIENT BRITISH VOLCANOES. 



Pre-Cambrian Volcanoes. 



Pre-Cambrian Volcano of St. David's. The oldest group of 

 rocks in this country, named pre-Cambrian, is largely volcanic. The 

 oldest portion, constituting the Dimetian formation of Dr. Hicks, 

 is a variable rock, regarded as intrusive syenite by Professor Kamsay, 

 and as intrusive granite by Dr. Archibald Geikie, in its typical 

 exhibition at St. David's ; and this part of the series is generally 

 admitted to be everywhere free from volcanic rocks ; but, so soon as 

 we pass above these ancient gneisses, with their interstratified schists 

 and limestones and granitoid rocks, and come to the series named by 

 Dr. Hicks Arvonian and Pebidian, we find proofs of remarkable 

 volcanic activity. It is possible that the so-called "Arvonian" 

 formation is only the lower part of the " Pebidian." It consists 

 chiefly of felsites of the type known as " halleflintas," with quartz 

 felsites and breccias. On the hypothesis of the so-called Dimetian 

 being intrusive, this felsitic condition of the rock immediately above 

 it is a matter of some interest, because it corresponds to the state in 

 which the underlying rock would appear in its outer portion in 

 consequence of rapid cooling, and a similar rock is found on the 

 margins of many important masses of granite. According to the views 

 of Dr. Hicks, these felsitic rocks are ancient rhyolitic lavas. The 

 overlying Pebidian series, however, consist to a large extent of 

 volcanic ash, interstratified with micaceous and chloritic schists. The 

 lowest beds include, in the agglomerates, spherulitic felstone lava ; 

 and higher up are many alternations of felstone and volcanic tuff. 

 Hence the whole series is precisely such as might have been fur- 

 nished if the granitoid rocks of St. David's had been the base of 

 an old volcano, and the materials ejected from it had sometimes 

 decomposed into muds and sometimes formed ordinary volcanic 

 rocks. 



In North Wales the three subdivisions of pre-Cambrian rocks 

 have been established by Dr. Hicks, Professor Hughes, and other 

 observers, and show similar characteristics. 



Pre-Cambrian Volcano of the Wrekin. Mr. Allport, who has 

 contributed greatly to establish the identity of volcanic rocks of the 

 Primary and Tertiary periods, notices a narrow ridge of igneous rock 

 which extends two miles and three-quarters through Ercal Hill and 

 the Wrekin. A second ridge lies to the west. Both these rocks 

 are highly acidic, and are associated with beds of volcanic ash 

 greatly indurated, which are well seen on the south of Lawrence 

 Hill. 



The lavas are reddish-brown felsite, or altered pitchstone. They 

 frequently exhibit on their weathered faces numerous parallel lines, 

 which sometimes show complicated folding. These lines indicate the 

 presence of streams of microliths, such as are seen in Hungarian 

 perlites of Tertiary age. Sometimes the spherulitic rock of the 

 Wrekin consists of bright-red spherulites, set in a grey or yellowish- 



