CHAP. LI 
SUMMARY AND GENERAL DEDUCTIONS 
475 
Table of the Periods of Volcanic Action in the British Isles and 
of the Chronological Distribution of the Volcanic Products. 
Granites, Grano- 
pliyres, etc. 
*“■< 03 ■ 
2P uT 
o 
% 
fc. 
a 
e . 
oT o tc 
- -g be 
c 
Trachytes. 1 
Andesites 
(Porphyrites). 
Gabbros. 
f sf 
£ 
- 
Q C 
r— p 
c ‘55 
lit 
3 To"" 
fit 2 
Tuffs, acid or 
basic. 
Older Tertiary 
(Plateaux, dykes, 
* 
•St 
* 
necks. Bosses, sills) 
* 
Mesozoic 
No volcanic rocks. 
Permian . 
* 
* 
* 
it 
* 
Carboniferous 
? 
* 
it 
it 
Pay type . , • 
* 
it 
it 
Plateau type . 
* 
it 
f Devonian 
•{ Old Red Sandstone 
it 
Upper . 
■it 
it 
I Lower . 
Silurian 
Upper 
■it 
it 
it 
* 
Lower, Bala . 
„ Arenig 
it 
* 
* 
* 
Cambrian . 
■St 
Uriconian 
* 
? 
Dalradian 
Torridonian 
Lewisian 
* 

* 
an eruptive mass after it lias been injected into its position within the 
crust of the earth. . . , , 
Prom the history of volcanic action in the British Isles it is c eai a 
differentiation is effected under three distinct conditions. _ . 
In the first place, a notable difference may he occasionally observe 
between two adjacent parts of the same mass of lava which ms ovve< ou 
at the surface. ' Thus, in the Carboniferous picrite of Blackburn, there has 
been a separation of the heavy basic constituents, which have m grea pai 
settled down into the lower part of the sheet, while the lighter ie spar nis 
mainly come to the top. In this case the gradual transition from top to 
bottom suggests that the separation occurred after the lava had reached 
the surface and taken the form of a stream or sheet. 
In the second place, segregation has taken place in the magma within the 
terrestrial crust after intrusion, for it is frequently observable m large bosses 
