~ Parr VIL Sect. i. § 2.] ERUPTIVE SHEETS. 547 
Connection with Volcanic Action.—There ean he little 
doubt that in regard to eruptive masses, particularly of the dioritic, 
diabasic, and doleritic or basaltic series, though the portions now 
visible consolidated under a greater or less depth of overlying 
material, they must in many cases have been directly connected with 
superficial volcanic action. Some of them may have been under- 
ground ramifications of the ascending molten rock which poured 
forth at the surface in streams of lava, though these superficial 
portions have been removed by denudation. Others may mark the 
position of intruded masses which were arrested in the unsuccessful 
attempt to open a new volcanic vent. 
§ 2. Sheets. 
Eruptive masses have been intruded between other rocks, and 
now appear as more or less regularly defined beds. In almost all 
cases it will be found that these intrusions have taken place between 
the planes of stratification. The ascending molten matter, after 
_ breaking across the rocks, or rather after ascending through fissures 
either previously formed or opened at the time of the outburst, has 
at last found its path of least resistance to lie along the bedding 
planes of the strata. Accordingly it has thrust itself between the 
beds, raising up the overlying mass and solidifying as a nearly or 
exactly parallel cake or sheet. 
It is evident that one of these intercalated sheets must present such 
points of resemblance to a stream of lava that flowed above ground 
as to make it occasionally a somewhat difficult matter to determine 
its true character, more especially when, owing to extensive denuda- 
_ tion, or other cause, only a small portion of the rock can now be seen. 
The following characters mark intrusive sheets, though they must not 
be supposed to be all present in every case. (1.) They do not rigidly 
- conform to the bedding of the rocks among which they are intercalated, 
but sometimes break across it and run along on another platform. 
(2.) They catch up and involve portions of the surrounding strata. 
(3.) They sometimes send veins into the rocks above and below them. 
(4.) They are connected with dykes or pipes which, descending 
through the rocks underneath, have been the channels by which 
the intrusive sheets were supplied. (5.) They are commonly most 
close-grained at their upper and under surfaces, and most coarsely 
erystalline in the central portions. (6.) They are rarely cellular or 
amygdaloidal. (7.) The rocks both above and below them are usually 
hardened and otherwise more or less altered. 
As a well-known and (from its association with the Huttonian and 
Wernerian disputes) classical example of this structure, the mural 
escarpment called Salisbury Crags at Edinburgh may be described. 
This is a sheet of crystalline dolerite which can be traced for a dis- 
tance of 1500 yards, lying among the red and grey sandstones, shales, 
and impure limestones, which form the base of the Carboniferous 
2N 2 
