438 PROFESSOR GEIKIE ON THE 
INTRODUCTION. 
The geographical area embraced in the present memoir forms a well-marked 
basin traversed along its centre by the estuary of the Forth. It is bounded on 
the north by the chain of the Ochil Hills, on the south by the range of the 
Pentland and Lammermuir uplands. Towards the west it joins along a low 
watershed the basin of the Clyde, while eastwards it dips under the waters of 
the North Sea. Within this defined space the Carboniferous rocks occupy 
what may be described as one great synclinal trough, varied by innumerable 
smaller synclines and anticlines. Save where cut out by powerful dislocations, 
their lower members rise up along the margins of the basin, while their highest 
portions cover a smaller area in the centre. The older formations forming the 
northern and southern boundaries of the area belong chiefly to the Lower Old 
Red Sandstone, in the Lammermuir district to the Lower Silurian. The 
Carboniferous rocks everywhere rest upon them unconformably. 
Within the region thus limited the Carboniferous system in its Scottish 
type is admirably displayed in numerous natural and artificial sections. Every 
great group of strata can be satisfactorily examined, and thus a thorough 
knowledge can be obtained of the detailed stratigraphy. One of the most 
noteworthy features in the geological structure of this part of Scotland is the 
abundance and variety of the volcanic rocks associated with the older half of 
the Carboniferous system. For a protracted geological period the site of the 
present basin of the Firth of Forth was occupied by numerous volcanic vents, 
emitting showers of tuff and streams of lava, which were duly interstratified 
_ with the contemporaneous sediments. A record has thus been preserved of a 
remarkable phase of paleeozoic volcanic activity. Affording means of compari- 
son with Tertiary and modern volcanic phenomena, down even into many points 
of minute detail, it enables us to decide whether volcanic action was essentially 
different in early geological times from its present conditions. Traces of the 
influence of WERNER’s doctrines regarding the modern and abnormal character 
of volcanic action may be detected even yet in modern geological literature. 
It has seemed to me that a useful end might be served by subjecting the well- 
developed volcanic series in the Carboniferous system of the Forth district to 
rigid scrutiny in the field and detailed study with the microscope. The results 
of this investigation I now lay before the Society. 
From the number and variety of the natural sections near Edinburgh the 
igneous rocks of this region have been the subject of numerous observations from 
the early days of geology down to the present time. The mere enumeration of 
the titles of the various publications on the subject would form a long list. As I 
