


GEOLOGICAL SURVEYING 301 
pipes of eruption. When these structures are seen either in 
plan or in section, their character is at once revealed. Some- 
times, however, the actual line of junction between them and 
the rocks they traverse is entirely concealed, and in such 
cases they might possibly be mistaken for outliers. Fig. 118, 
for example, shows in ground-plan field data which are so 
apparently incomplete that the agglomerate and tuff a, might 
be explained as an outlier resting unconformably upon the 
truncated ends of the strata 4. We should have no doubt, 
however, as to its intrusive nature if we could make the 
following observations :—1. The tuff either shows no bedded 
arrangement, or, if any trace of bedding be visible, the dip of 
the rude layers is towards the centre of the mass; 2. Dykes 
or veins of crystalline rock traverse the tuff, while similar 
veins of the same rock appear at some little distance invading 

Fic. 118.—GROUND-PLAN OF NECK. 
Continuous lines and arrows= boundaries and dips exposed in section. Interrupted lines= 
conjectural positions of boundaries. Stippling, etc.=rocks exposed at surface. 
Gie adjacent rocks; 3. Ihe surrounding strata as they 
approach the tuff are more or less shattered, and perhaps 
show traces of induration as if from the action of heat. 
Should the portions exposed be not far from the concealed 
junction, the beds may appear suddenly to bend over so as to 
dip abruptly towards the agglomerate or tuff; 4. Fragments 
of the adjoining rocks and of rocks which may be recognised 
as belonging to lower and higher geological horizons, can be 
detected in the tuff. Evidence of this varied kind would 
satisfy us that the igneous rock was not an outlicr but a 
neck, and we should be justified in drawing around it an 
interrupted line. 



