562 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. | 
though even they have undergone more or less modification, the 
sandstone being converted into vitreous quartz-rock.* 














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Fic. 290.—Srcrion across THE SALINE HILs, Fire. 
mass has rolled 
poraneously formed sedimentary strata; 
e smaller eminence, or Knock Hill, while 
t, Continuation of tuff of cone intercalated with the contem 
The thick parallel lines are coal-seams which are burnt round th 
they can be worked for some way under the larger, or Saline Hill. 
T, Tuff of necks; 
B, Basalt 
Section II. Interbedded Volcanic or 
Contemporaneous Phase of Erup- 
tivity. 
Masses of igneous materials, ejected to 
the surface in some of the forms now ~ 
visible in modern volcanoes, possess great 
value as fixing the geological epoch of 
volcanic eruptions. It is evident that on 
the whole such superficial masses must 
agree in lithological characters with rocks 
already described, which have been ex- 
travasated by volcanic efforts without quite 
reaching the surface. Yet they have some 
well-marked general characters, of which 
the most important may be thus stated. 
(1.) They occur as beds or sheets, some- 
times of lava-form, sometimes of frag- 
mental materials, which conform to the 
bedding of the strata among which they 
are intercalated. (2.) They do not break 
into or alter overlying strata. (3.) The 
upper and under surfaces of the lava- 
beds present commonly a scoriaceous or 
vesicular character, which may even be 
found extending throughout the whole of 
a sheet. (4.) Fragments of these upper 
surfaces not unusually occur in the imme- 
diately overlying strata. (5.) Beds of 
tuff are frequently interstratified wit 
sheets of lava. ! 
§ 1. Crystalline, or Lavas. 
While the underground course of a 
protruded mass of molten igneous rock 
has widely varied according to the shape 
of the channel through which it proceeded — 
and in which, as in a mould, it solidified, 
the behaviour of the rock, once poured 
out at the surface, has been much more 
uniform. As in modern lava, the erupted 
along, varying in thickness and other minor 
1 For a detailed account of the structure of some volcanic necks the student may 
consult a monograph by the author on the Carboniferous volcanic rocks of the Basin of 
the Forth. Trans. Roy. Soc. Edin, xxix. p, 487. 
