S0J1E ANCIENT. AND MODEEN VOLCANIC EOCHS. 



417 







01 



" a 2 



Is 





•i-gS 



03 



•H 



>> 



' & 

 O 





s ° 



o 



sip 



32J 



Silica .. 



48-02 



20-78 



7-12 



3-65 



10-18 



1-16 



7-97 



mean 



51-0 



14-0 



0-2 



3-4 



10-0 



5-5 



14-7 



max. 

 55 

 16 



1 



5 

 13 



9 



18 



60-718 



14-894 



2-354 



2-843 



6-048 



1-909 



6-426 



1-405 



•395 



•281 



•103 



1-660 



•964 



mean 



61-5 

 15-5 

 2-5 

 2-5 

 3-5 

 1-5 



7-5 



mean 



72-8 

 15-3 

 6-4 

 1-4 

 0-7 

 0-9 



1-7 



mean 



75-4 

 15-0 

 3-1 

 1-3 

 0-8 

 1-1 



2-3 



83-802 



7-686 



2-161 



4-229 



•896 



•109 



•408 



•111 



•191 



•089 



•017 



trace 



•301 



69-673 



14-492 



4-554 



3-017 



2-296 



•324 



2-784 



•442 



•410 



•343 



•205 



•660 



•800 



Alumina 



Potash 



Soda 



Lime 



Magnesia .. 



Ferrous oxide 1 



Ferric oxide J 



Bisulphide of Iron ... 



Phosphoric acid 















Sulphuric aci d 















Carbonic acid 















Loss on ignition 





1-1 



3 







0-8 



1-0 







If these rocks do not fall strictly under any recognized name, I 

 would suggest that for the present they should be regarded as inter- 

 mediate forms similar to the trachy-dolerites, and therefore they 

 might be included under the name Felsi-dolerite, denoting a com- 

 bination of felsitic and doleritic characters. To such I would give 

 the following provisional and general definition: — 



Felsi- (Trachy-)dolerite. — Silica 59-61 per cent. Matrix generally 

 crystalline, containing crystals of labradorite or oligoclase, and ortho- 

 clase, porphyritically imbedded, round which the small crystalline 

 needles seem frequently to have flowed ; magnetite generally abun- 

 dant, and augite tolerably so, though usually changed into a soft 

 dark-green mineral ; apatite, and perhaps olivine, as occasional con- 

 stituents. The rock usually of some shade of blue or dark green, and 

 generally weathering white round the edges, but to a very slight 

 depth ; it frequently assumes a tabular structure, the tabulge being 

 often curved, and breaks with a sharp, conchoidal, and flinty frac- 

 ture. 



g. Conclusion. — In conclusion, I would again point to the intense 

 amount of alteration these old lavas have undergone, obliterating 

 much of their original character. Many of these rocks, especially 

 some of the volcanic centres, such as Castle Head, Keswick, might 

 be called diabase, chlorite being a constant constituent. Mr. Allport 

 has traced true dolerites as far back as Carboniferous times ; and it 

 may be that in rocks so much older, as are these of Lower Silurian 

 age, there will he found — owing perhaps to greater metamorphism — 

 no lavas with a closer likeness to modern basalts than those of Cum- 

 berland, just as the ancient felstone lavas are sufficiently distinct from 

 their modern representatives the trachytes. 



Be this as it may, the undoubted fact remains that in Lower Silu- 

 rian times there existed in Cumberland volcanoes of sufficient mag- 

 nitude, and remaining active for a sufficient period of time, to accu- 

 mulate a thickness of at least 12,000 feet of volcanic products. This 



2f2 



