458 REMNANTS OF EXTINCT VOLCANOES. 



rocks, which consist of coarse grained dyke gabbro, and banded 

 gabbro, coarse and fine grained, and serpentine bands in the 

 various stages of change from perfect olivine grains, enstatite 

 and bronzite, into a fine workable serj)entine. Traces of the 

 original crystaline structure are still discernable, these rocks 

 being much contorted at their junction with the lower group, 

 their average dip being about 44° N. A much more complete and 

 satisfactory non-conformity of matter, strike, and dip could not 

 be well conceived by the most fertile imagination. Further 

 evidence is adduced too, by the abundance of the north and 

 south faults or fissures, which are found crossing the rock west 

 of a line drawn from Abergele, in North Wales, to Exeter, in 

 the south of England. These cross-courses or fissures may be 

 attributed to the Tertiary convulsions which passed along in this 

 direction, and in some localities they were probably the vents 

 through which passed matter that formed huge mountains like 

 Etna and Vesuvius of to-day, but have shared a similar fate to 

 the denuded mountains of the western islands of Scotland. 



In some places the fissures show great displacement, one 

 which passes from Perranporth on the north, to near Ealmouth 

 on the south, was reported by the miners at "Wheal Leisure, 

 Perranporth, to show a displacement of 60 fathoms. 



This fissure is to be seen at an advantage about mid-way in 

 the eastern cliffs at Perranporth, its course being a little west 

 of north and east of south. Somewhat similar fissures between 

 Truro and Eedruth are very abundant, as if this were about the 

 central line of convulsions, or at least one of them, that dis- 

 turbed the rocks which had had long repose ; some of these faults 

 or fissures are fathoms in breadth, and are principally filled with 

 quartz, and are in some places metalliferous. This disturbance, 

 however unpleasant to the marine creatures which gained their 

 subsistence there, in course of time proved beneficial to man, 

 whose remains have been found in deposits at Carnon and 

 Pentewan tin stream works. 



Now let us take a brief review of the past within Tertiary 

 times : what must have been then the condition of the region 

 now called England, when Snowdon was a favourite resort of 



