634 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
certain organic type, and referable to a certain place in the scale of geological time. 
Hence, too, when he comes to a district abounding in igneous rocks, he often 
finds it insufficient to separate them out into augitic, or felspathic, or hornblendic, 
as the case may be, for such a subdivision tells nothing of the relations in time 
of these rocks, to the stratified deposits with which they are associated. Over 
and above this mineralogical distribution, another must be adopted, which shall 
enable him, as far as possible, to throw each igneous mass into its true chronolo- 
gical place, that in this way the succession of events in the geological history of 
the district may be clearly and methodically detailed. 
In carrying on a detailed examination of the trap-rocks throughout a consider- 
able part of the country, [ have found by much the best classification for practice 
in the field to be that adopted by the Geological Survey throughout the trappean 
region of Wales. There the traps were divided into,—1s¢, Felspathic ash and ashy 
sandstones, and slates; 2d, Interbedded felspathic rocks; 3d, Intrusive felspathic 
rocks; and, 4¢, Greenstones and basalts. In extending this Welsh type into 
Scotland, it is necessary to subdivide the greenstones and basalts, as the felstones 
were subdivided, into two groups—interbedded and intrusive. The arrangement 
is thus exceedingly simple. There are, jirst, the ashy rocks, as to the origin and 
age of which we can seldom be at a loss, and which can be modified in the 
colouring of the map according to their composition. Then come the two great 
groups of the melted traps,—the felspathic and the augitic or hornblendic. The 
former are arranged, according to their mode of occurrence (which points of course 
to their relative age), into interbedded or contemporaneous, and intrusive or subse- 
sequent felstones. The latter group is similarly classified into ¢nterbedded or con- 
temporancous, and intrusive or subsequent greenstones and basalts.* 
It may be well to state here very briefly, the characters on which this classi- 
fication rests. ‘lt heashes, representing showers of volcanic dust and cinders which 
settled down upon the sea-bottom, or on the surface of the land in more or less 
stratified layers, yield by much the clearest evidence as to the age of any igneous 
eruption, inasmuch as their position in a series of rocks marks the relative date of 
their production precisely, as though they were beds of sandstone or limestone, 
while, in addition to this, they in many cases, contain fossil remains, both 
vegetable and animal, which still further contribute to define their geological 
horizon. 
The truly interbedded melted traps are regarded as contemporaneous, that is, 
of the same age as the strata among which they lie as interstratified conformable 
* In the colouring of the maps of the Geological Survey, this arrangement is followed, by using 
crimson for the augitic, and a brighter red for the felspathic traps. The dark shade of each colour 
marks the intrusive masses, while the lighter shade is used for the interbedded sheets. The age 
of the latter, as well as of the ash-beds, is always shown by the geological position of the strata 
with which they are associated. 

