GEOLOGICAL SURVEYING 281 
instead of writing sandstones and shales, SS or Sa & Sh will 
suffice. In like manner, most of the common igneous rocks 
can be indicated by means of the initial letters, as B for 
basalt, G for granite, Sy for syenite, D for dolerite, Dz for 
diorite, P for porphyrite, and so on. Plate LI. shows some 
of the signs and symbols used by the Geological Survey of 
Great Britain and Ireland. 
Tracing Exposed Outcrops.—As the most continuous 
exposures of rock naturally occur upon sea-coasts and along 
river-courses, it is best for practice to select, if possible, some 
tract the situation and topography of which seem to promise 
the observer most information. Proceeding along the sea- 
coast, and following the stream-courses of a region which we 
shall suppose consists largely of stratified rocks, the student 
must insert upon his map the direction and angle of dip 
as frequently as possible. The outcrops of all notable or 
important beds and seams (such as limestones, coals, iron- 
stones, etc.) are carefully set down, and particular descriptions 
of these and the accompanying strata are recorded in the 
note-book. Fossils are sedulously searched for everywhere, 
more particularly in the finer grained argillaceous sandstones 
and shales amongst which seams of coal and ironstone or 
beds of limestone not infrequently occur. Should any seam 
or layer be characterised by the presence of certain fossils 
peculiar to itself, the exact position of such seam should be 
carefully indicated, for it may be of great service as a datum- 
line or geological horizon, as will be shown presently. 
Bedded ironstones and limestones are often marked by the 
presence of special fossil-forms, and this is one reason why 
the outcrops of such rocks are invariably mapped by a field- 
geologist. Any stratum or series of strata, however, which 
may be notable on account of fossils or lithological character, 
must be distinguished from immediately overlying and under- 
lying strata. Not infrequently it is possible to separate a 
great succession of sedimentary deposits into subordinate 
groups—each, it may be, marked by the presence of particular 
fossils, or by the composition and structure of the rocks 
themselves. 
Tracing Concealed Outcrops.—After the observer has 
examined every exposure of rock upon the sea-coast, in river- 


