4 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
Balthayock, Pitroddie, Abernyte, Fingask, Kilspindie, Craigneb 
Evelick, and Dogballo. 
An inspection of the Rossie Quarry presents some points of in¬ 
terest which may be noticed. The quarry is situated on an exposed 
upland, and hence has been subjected to excessive decay in its upper 
portion. The dark green rock underneath is overlaid by a weathered 
crust, 30 feet in depth, and in some parts the percolating water has 
penetrated to still greater depths. The silicates of the rock may on- 
these upper portions be observed in all the stages of decay from the 
friable earth to the true rock underneath. Now, an inspection of 
the rock mass of the trap dyke in the Corsiehill Quarry shows the 
dominant divisional lines which all igneous rocks naturally form when 
contracting in a horizontal position—that is, at right angles to the 
cooling planes. 
Again, an inspection of the Rossie Quarry will at once show^ that 
the rock is differentiated from such eruptive rock as the trap dykes, 
so common in Perthshire and the Tay valley, by the system of 
prismatic joints. The position of these joints, by which all volcanic 
rocks are traversed, is often sufficient to indicate whether a rock is 
intruded as a dyke or neck or erupted on the surface as a lava flow. 
These cooling planes, or joints, start from the cooling surface inwards. 
In an erupted mass they will be perpendicular to its upper and under 
surface, while in a dyke they will be horizontal when the dyke is 
vertical. The trap dyke to be seen in Corsiehill Quarry presents 
these horizontal cooling planes to perfection. The cooling planes of 
the Rossie Rock are perpendicular, or approximately so. In blasting, 
the rock mass gives way along these joints, and some parts of the 
quarry are perpendicular and smooth like the painted wall of some 
huge building. To designate the face of the quarry as painted is no 
figure of speech or exaggeration of facts. The basalt is very rich in 
olivine, and in the course of ages the decomposition products of the 
whole rock mass, but chiefly of the constituent olivine, have been 
deposited along these joints, just as the silicates of an agate, or 
Scotch pebble, are deposited as a decomposition product on the 
inside of the round steam vesicles of the porphyrites. 
As already stated, along the vertical joints, or cracks, water has 
percolated for ages, carrying with it in solution the mineral con¬ 
stituents of the rock itself These constituents may be deposited as 
a segregated mass or left as a cr3^stallized incrustation on the cavity 
walls. The joints so filled with these segregated minerals serve to 
fill up what would otherwise be empty veins. The decomposition 
products serve to bind the rock together with a crystalline cement 
produced from its own decay, and so prevent the whole rock mass 
becoming a ruinous heap from earth movements. It is a matter of 
