British Billow-lavas. 205 



must be remembered tbese are only the westward prolongation of 

 a great series of pillow-lavas (13) (with attendant keratophyres, 

 minverites, etc.) that extend from Moravia through Nassau across the 

 llhine into England. Many accounts of these Devonian schalsteins 

 have been furnished by Continental petrographers. Some have 

 noticed their richness in soda, but other felspars are often described 

 as characterizing this group of rocks. Many of them contain fresh 

 augite and labradorite or andesine, in others the felspar is partly 

 oligoclase. The Devonian spilites of the West of England contain 

 no pyroxene and no felspar but albite. Yet there can be no doubt 

 that they are the effusive rocks of one eruptive period, furnishing 

 one of the best examples of a petrographical province that could 

 be adduced. Hence, as the most decomposed rocks contain most 

 albite it seems reasonable to assume that the mineral is in some way 

 of secondary origin. 



In these rocks albite occurs sometimes in vesicles and veins, but 

 the minerals of the amygdules are mostly calcite, chlorite, and quartz. 

 The highest lavas of the series are interbedded in the Lower Carboni- 

 ferous. At the close of the Carboniferous period the Armorican earth- 

 movements began. The spilites were crushed and, when highly 

 vesicular, were torn in shreds. Their felspars are broken and twisted, 

 and the evidence of the slides proves that before the movements set in 

 the albitization and decomposition were complete, for the broken albites 

 are not healed or their fractured surfaces united by new deposit. 



Shortly after the movements ceased (perhaps before they were 

 completely at an end) great eruptive granites, of which the Dartmoor 

 mass is the best known, invaded the region. The lavas were at that 

 time rotten and sheared, and their vesicles filled with secondary 

 minerals. "Within the aureoles of the granite bosses they were now 

 hornfelsed and their albite felspar replaced by andesine and labradorite, 

 while their calcite and chlorite gave rise to epidote, augite, biotite, 

 and hornblende. The basic felspar can be traced in every stage of 

 development as we follow the spilitic lavas from the outer precincts of 

 the aureoles to the actual contacts with the granite ; and that the 

 felspar is recrystallized from a mineral that was full of secondary 

 products can be proved by the abundance in it of specks of horn- 

 blende, augite, and biotite, that gradually coalesce to larger grains as 

 the alteration becomes more intense. The new basic felspar produced 

 in this way in late Carboniferous times is still perfectly fresh and 

 clear ; this proves that some factor other than mere lapse of time with 

 concomitant weathering must be regarded as the prime agency of 

 albitization in this case. 



To show the chemical composition of these rocks we quote some 

 analyses that have been prepared in the laboratory of the Geological 

 Survey. These demonstrate clearly the essential characters of the 

 group : they are basic rocks much decomposed and rich in carbonates 

 and combined water. In all cases the soda is high for basic rocks, 

 while the potash is very low. If the carbonic acid of the first, 

 second, and third analyses is assumed to be combined with lime to- 

 forni calcite, an assumption that has been verified by chemical tests, 

 the amount of lime felspar present must be quite negligible. 



