486 PROFESSOR GEIKIE ON THE 
Part IJ.—PETROGRAPHY. 
In the following section of this Memoir I propose to offer a sketch of the 
general results of a somewhat extended investigation of the characters revealed 
by the microscope in the various igneous rocks of the Basin of the Firth of 
Forth. A few of the more accessible rocks of the region have been micro- 
scopically examined and described by several observers, more especially by Mr 
ALLPORT, in whose interesting communication on British Carboniferous 
dolerites an account in particular is given of some of the igneous masses round 
Edinburgh.* But no attempt has yet been made to compare the minute 
structure of the volcanic rocks as a whole, and in connection with the history 
of volcanic action in the region, and to determine the leading types of 
composition and arrangement. 
During the last twelve years I have devoted much time to the study of the 
microscopic characters of the crystalline rocks of Scotland. One portion of this 
inquiry has embraced an examination of the volcanic rocks described in the 
foregoing part of the present Memoir. I have had thin sections prepared of 
about 350 rocks from all parts of the region, illustrating every variety of 
composition and texture encountered by me in the field.t These sections 
present a most instructive picture of the original diversities of the volcanic 
masses, as well as of the alterations which their internal structure has subse- 
quently undergone. The details which they furnish are full of interest, and I 
hope to publish them elsewhere. But a resumé of the chief results as yet 
arrived at in the course of the investigation may appropriately find a place as a 
sequel to the stratigraphical narrative in the preceding pages. ; | 
“ the purpose of petrographical classification, as well as of stratigraphical _ 
detaif, voleanic rocks may be arranged in two great leading subdivisions. 
I. The Crystalline, embracing all which have been erupted to the surface or 
intruded below in a molten condition ; and II. The Fragmental, including all 
which have been thrown out in a fragmentary form. The former may be termed 
briefly the Lavas, and the latter the Tuffs. > | 
I. THE CRYSTALLINE Rocks or LAVAS. 
Looked at in their petrographical aspect as they occur among the Carboni- — 
ferous formations of the Basin of the Firth of Forth, the Crystalline rocks or 
Lavas may be arranged into four chief groups—1st, Augite-felspar Rocks, — 
* “Quart. Journ. Geol. Soc.” vol. xxx. p. 529. 7 
+ These sections have been excellently prepared, in the petrographical laboratory of the Geologie 
Survey of Scotland, by A. Macconocure and R. Luny. 
> 
2. 
