492 PROFESSOR GEIKIE ON THE 
From the description now given it will be seen that the largely crystalline 
intrusive sheets in the Basin of the Firth of Forth possess characters which do 
not admit of their being classed without some qualification in any established 
petrographical genus. From the dolerites they are marked off by the frequent 
preponderance of orthoclase and the presence of free quartz. From normal 
diabase they are also distinguished by their proportion of orthoclase, though 
undoubtedly they approach most closely to this rock, and I have retained this 
name as a generic designation for them. In many respects they remind one of 
augite-andesite. They show a similar mixture of monoclinic and _ triclinic 
felspars with augite, and a sparing quantity of free quartz. In judging of their 
relations to other rocks we must remember that they are always intrusive 
sheets, that in many cases they have caught up, and seem actually to have 
dissolved into their substance portions of the rocks through which they have 
been injected, and that as a fact they show very considerable differences of 
composition even within short spaces inthe same mass. The proportions of the 
felspars, the relative abundance of the titaniferous iron and augite, the size and 
number of the cavities filled with alteration-products, all vary greatly from point 
to point. 
Bearing these gradations in mind, we cannot be surprised to find that no 
line of demarcation can be drawn between the coarsely-crystalline orthoclase- 
bearing diabases and the dolerites to be immediately described. Varieties of 
rock occur which, according to the feature in their composition most kept im _ 
view, might be referred to either group. 
IJ. Tue Douerires. (Plate XI. Fig. 3.) 
Under this title I have since the year 1867 classed the dark crystalline- 
granular augitic sheets by which the Carboniferous rocks of central Scotland 
have been invaded, and which were previously embraced under the term 
“ oreenstone.”* Though the word “ dolerite” has been restricted by German 
petrographers to rocks of Tertiary and post-Tertiary date, I was unable to- 
discover any difference between those associated with the Carboniferous rocks 
and those which in the Inner Hebrides are certainly of Tertiary age. The - 
opportunities for a comparison between them were many and favourable. I 
traced the probable connection of the numerous east and west “ trap-dykes” 
which traverse Scotland, with the great Miocene lava-plateaux of the Inner 
Hebrides, and showed that these dykes, cutting as they do through all the 
known paleozoic and secondary rocks, including parts of the Tertiary volcanic 
sheets, and even across the latest faults of the country, were almost certainly of 
* Sheet 14, “ Geological Survey, Scotland, Dec. 1868,” an] Explanation of same. 
