THE ROCKS OF PLYMOUTH. 219 
various points between the clay-slate and the limestone. Both 
these examples are probably mechanical admixtures, the result 
of casual association during formation. 
PRELIMINARY NOTES ON IGNEOUS ROCKS. 
Several attempts have been made to classify igneous rocks by 
this presumed origin, but none have proved satisfactory. They 
have for example been divided into plutonic, those which have 
been formed in the depths of the earth’s crust; and volcanic, 
those which have consolidated at the surface. But many once 
plutonic rocks are now by denudation exposed to view, and the 
distinction is really one of degree. Again, igneous rocks of the 
older formations have been regarded as different in character to 
those of the newer. But the more closely the series is studied 
the less important and distinctive does this question of age 
appear. Then structural peculiarities have been relied upon. 
But these, though valuable in their way, have rather a casual 
value; for rocks of the same age, and practically of the same 
composition, under various conditions assume very different 
aspects. All things considered, the most important features to 
be noted are, first, the mineral composition of an igneous rock ; 
and, second, the fact of the rock being contemporaneous with or 
intrusive among the strata with which it is associated. Structural 
differences have great weight, but it is commonly pathological. 
These observations will explain the vagueness of some of the 
terms current among geologists in dealing with rocks of igneous 
origin. The words “greenstone” and “trap” are found used 
interchangeably to cover nearly all the older species, the granites 
and their allies excepted. There is a certain value in the 
possession of phrases which may be employed without expressing 
any definite opinion, but their employment is apt to mislead the 
uninitiated. Especially is this so when we find, as on the 
geological map of the district, igneous rocks of the most dis- 
similar characters coloured and classed as “‘ greenstone.” 
Our greenstones or traps really belong to six distinct classes : 
1, A large body of ancient volcanic rocks—the dunstones— 
contemporaneous with the rocks among which they are found, 
therefore of Devonian age. These are the oldest igneous rocks in 
the Plymouth area, and there seems good reason for associating 
