354 British Voleanie Recks. [July, 
may be induced to discuss it practically for themselves in the field. 
Avoiding detail, therefore, as far as possible, let me endeavour to 
indicate, first, what volcanic rocks are, and how they are to be 
detected ; and secondly, that we have numerous examples of them 
in Britain of many different geological ages. 
I, All igneous rocks are not necessarily volcanic. In many 
cases masses of melted matter haye been injected from below into 
the crust of the earth deep beneath the surface. It is only where 
the erupted material has been thrown out at the surface that it 
properly takes the name of volcanic. Hence in deciphering the 
geological structure and history of a country care must be taken 
not to misapply that term. An obvious classification of volcanic 
rocks is imto two divisions:—1l. The lava-form series, or those 
which have cooled down from the state of lava; and 2. The ashy 
series, or those ejected in the form of loose material, such as stones, 
cinders, dust, and ashes, which have often accumulated in thick 
masses both on land and under water. Each group is further sub- 
divided according to the composition or structure of its rocks, but 
into these details we need not here enter further than to note that 
varieties of our old lavas are known as basalt, greenstone, clinkstone, 
felstone, or more generally as trap ; while the ashy series includes 
ash or trap tuff, volcanic breccia, and agglomerate. These rocks 
may evidently be studied under two very different aspects. They 
may be viewed either as so many mineral products coming from the 
depths of the earth’s crust, upon the composition of which they may 
be expected to throw some light; or they may be looked upon as 
memorials of changes in the geological history of the country. 
Regarded in the latter light, our first object is to search for evidence 
that they are truly volcanic, and not merely masses which have 
been intruded into later rocks and cooled deep below the surface. 
It is evident that the occurrence of layers of ash or tuffis a sufficient 
demonstration that the rocks under examination were erupted either 
under water or in the open air, and must be of volcanic origin. For 
we cannot conceive of the formation of beds of such loose matter 
within the crust of the earth. Ash or tuff is usually an easily recog- 
nizable rock. It consists of a paste of comminuted trap with more 
or less intermixture of ordinary sandy or muddy sediment. Some- 
times itis nothing more than such a fine paste, but it often contains 
an admixture of fragments of trap and other rocks varying in size 
from mere grains up to blocks several feet in diameter. Hence there 
are gradations from the finest ash, through gravelly tuff and breccia, 
into the coarsest agglomerate. As a rule, the coarser the material 
and the less mixed it is with ordinary sediment, the nearer probably 
does it lie to the original focus of eruption. Im many ashes or 
ashy beds organic remains are abundant, and from these we learn 
that the volcanic dust was showered down upon the sea, on lakes, 
