
300 STRUCTURAL AND FIELD GEOLOGY 

these, drawing his lines so as to show the shape of the mass— 
whether it be circular, elliptical, or quite irregular, or following 
-in a rude way the strike of the surrounding rocks. The 
numerous veins, etc., must be generalised, but when well 
exposed in section or in plan, it is advisable to make careful 
drawings of these for future reference, when the phenomena 
come to be described. So far as he can do so, the observer 
will try to indicate upon his map the nature of the altered 
rocks which surround the granite. The stages of contact- 
metamorphism, however, so frequently graduate into each 
other, that it is often quite impossible to draw boundary-lines 
separating one kind of metamorphic rock from another. 
Nevertheless this can sometimes be done, more especially in 
cases where the original unaltered rocks have differed 
markedly in character, and have thus been metamorphosed 
into more or less strongly contrasted sub-crystalline and 
crystalline rocks. There are many other observations that 
the field-geologist will find it impossible to indicate upon a 
map, but which he should not fail to describe in his note- 
book. 
Szls are not, as a rule, hard to trace. Even when the 
actual] lines of junction with adjacent rocks are not exposed, 
the intrusive character of a sill is frequently indicated by 
the way in which it seems to steal across the strike of the 
strata. The absence of any bedded tuff accompanying the 
igneous rock, would be so far suggestive of the intrusive 
character of the latter. This negative evidence, however, 
would be much strengthened if veins of the same kind of 
rock were found penetrating the overlying strata. We could 
hardly doubt in that case that the veins were genetically 
connected with the igneous rock, and that the latter therefore 
was not truly bedded, but an intrusive sheet or sill. Dykes 
are even more readily diagnosed in the field than sills, and 
can usually be followed without difficulty. Their presence is 
often revealed by lines of springs which come to the surface 
on that side of a dyke towards which the strata are inclined. 
For the various details of structure and the general pheno- 
mena characteristic of sills and dykes, however, reference 
should be made to Chapters XIII. and XIV. 
The same chapters also give some account of Vecks or 
