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548 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
system of central Scotland. As the general dip of the rocks is north- 
easterly, it forms a lofty cliff facing west and south, from the base of 
which a long grassy slope of débris stretches down to the valley in 
front; the thickness of the sheet at the highest part of the bed is about 
80 feet, but at a distance of 650 yards to the north this thickness dimin- 
ishes to less than a half. At first the dolerite might be taken for a 
conformable sheet, regularly interposed between the sedimentary strata. 
But an examination of the beds on which it rests shows that it trans- 
gressively passes over a succession of platforms, and eventually comes to 
rest at the east end on strata somewhat lower in geological position 
than those at the north end, Moreover, another parallel intrusive 




Fic. 274.—DIAGRAMMATIC VIEW OF SALISBURY CraAGs, EDINBURGH. 
sheet intercalated in a lower portion of the sandstone series gradually 
approaches the rock of Salisbury Crags. They are both transgressive 
across the strata, and they appear to unite in a large mass called 
Samson’s Ribs. 
On the west front a large dyke-like mass of the eruptive rock 
descends vertically through the sandstones, and has been regarded as 
not improbably a pipe or feeder, from which the molten rock originally 
rose (Fig. 274). Along the southern face of the escarpment several in- 
structive exposures show the behaviour of the dolerite to the strata through 
which it has made its way. Fig. 275, for example, represents a portion 

Fic. 275,—SEcTIon av BAsE or Sourn Fronr or Sauispury Craas. 
Showing portion of strata cut out by intrusive Dolerite. a, sandstones, shales, &c. ; 
b, dolerite. Length of section 22 feet, 
of the underlying strata carried away, the dolerite having been 
wedged in below one of the remaining broken ends. Again, veins and 
threads of the eruptive rock have been injected into fragments of the 
strata, caught up in its mass (Fig. 276). The strata in contact with the 

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