THE HIGHLAND BORDER ROCKS OF THE ABERFOYLE DISTRICT. 
201 
consist of quartz, microcline, orthoclase, oligoclase, muscovite and biotite, accom- 
panied by abundant zircons and widely disseminated green tourmaline. The Margie 
Grits are richer in spilitic fragments, the Leny beds on the whole decidedly richer 
in clastic micas. In neither case has a single pebble or grain of a schistose rock 
been noted. 
The associated mineral grains, taken together with the occasional occurrence of 
granite fragments, point to an area of granitic rocks as the ultimate source of the 
materials of both sets of grits. The occurrence of biotite and the fresh character 
of the soda-lime felspars suggest that the grits, which in both Series include types 
approaching arkoses, have been derived directly from such a source. The absence 
of schistose rocks, although a negative character, is noteworthy, since it implies in 
all probability the absence of such rocks at the surface in the region which was 
undergoing denudation. 
The Leny Grits, on the whole, exhibit a higher degree of metamorphism. This 
appears most obviously in the frequent occurrence of pronounced foliation, accom- 
panied often by a stronger development of authigenous green chlorite and quartz. 
In no case, however, has the clastic character been obliterated ; and, indeed, it is 
possible to obtain from the Leny Series grits which show as little trace of meta- 
morphism as the least altered Margie Grits. The argillaceous sediments of the Leny 
Series are characterised by well-marked slaty cleavage. 
The above notes on the Leny Series are based on an examination of specimens 
collected during a number of traverses over the area immediately to the north of 
the Leny Boundary Fault. Setting aside the slight difference in the degree of 
metamorphism, and considering only the character of the original constituents, we 
confess that the data so far obtained do not do away with the difficulty which 
previous observers experienced in attempting 'to distinguish lithologically the grits 
of the Border Series from the grits to the north. The sediments of the Leny Series 
have not, so far, yielded fossils ; the available petrographical evidence, although it 
suggests that the grits are an integral part of the Margie Series, is not conclusive ; 
the age of the beds, therefore, may still be regarded as an open question. 
XI. Later Intrusions. 
In the area between the Leny Fault and the Highland Boundary Fault the post- 
Ordovician intrusive rocks include (a) Porphyrites of Lower Old Bed Sandstone age, 
and (6) Quartz Dolerites which were intruded possibly in late Carboniferous times. 
The larger of two porphyrite intrusions occurs opposite Lime Hill, extending 
for a distance of about half a mile in the direction of the strike of the beds of the 
surrounding Old Bed Sandstone. Basic labradorite, usually with a narrow outer 
zone of more acid plagioclase, and ferro-magnesian minerals, including bastite 
pseudomorphs after hypersthene, chlorite and calcite replacing monoclinic pyroxene, 
and iddingsite after olivine, all appear as phenocrysts. The nature of the ground 
