ANCIENT VOLCANIC ROCKS. 6r 



of the felsites and associated rocks of Marble Head in 

 Massachusetts, which Wadsworth had recognised two years 

 earlier as devitrified rhyolites and altered ashes ("poro- 

 dites " ). More recently some advance has been made in 

 the study of the old volcanic formations of other tracts. 



To G. H. Williams (i 2) we owe a clear account of some 

 of the ancient lavas of the Appalachian region as developed 

 at South Mountain in Pennsylvania and Maryland. The 

 majority of the rocks are old rhyolites, and, in spite of 

 secondary changes and frequent crushing, the author re- 

 cognises not only the flow and banded structures, but spheru- 

 lites, lithophyses, pumiceous modifications, and other well- 

 known features of acid lavas. The rocks carry porphyritic 

 quartz and an alkali-felspar, but rarely any ferromagnesian 

 mineral. The ground-mass is a mosaic of quartz and fel- 

 spar, but that this is often due to devitrification and re- 

 crystallisation is proved by the preservation in many 

 instances of the characteristic structures of the acid glasses. 

 Basalts are also found, retaining their original structures, 

 ophitic, etc., and skeleton-crystals of olivine, but usually 

 more weathered and more crushed than the rhyolites. In 

 both types of lava epidote is one of the dominant secondary 

 minerals, and in the rhyolites it is in great part the man- 

 ganese-epidote, piedmontite. The lavas are accompanied 

 by fine banded ashes,, flow- and tuff-breccias, pumiceous 

 bombs, and other concomitants of ordinary volcanic erup- 

 tions. The whole group underlies strata containing a 

 Lower Cambrian [Olenellus) fauna. 



In a later paper Williams (13) extends his observations, 

 and describes typical devitrified obsidians, rhyolites, breccias, 

 etc., from Maine in the north and from North Carolina in 

 the south. From a general review of the literature of the 

 subject, he shows the high probability that such ancient 

 lavas and pyroclastic rocks will be found to occur throughout 

 the whole region from Eastern Canada and Newfoundland 

 to the Carolinas and Georgia. In numerous cases, although 

 no detailed descriptions have been given, geologists have 

 recognised the igneous, and sometimes the volcanic, nature 

 of the rocks. In other cases there is reason to believe that 



