John MacCulloch

“ If Nature has really produced imitations of mechanical arrangement by processes unknown to us, it is to be wished that the mode in which they have been produced may be shown, either by means of experiments, or by analogies drawn from that science of which the laws regulate the great proceedings of Nature as they do the narrow operations of our own laboratories. ”

“ When calcareous spar forms the amygdaloidal nodule, it is commonly invested with a coating of chlorite. These nodules are generally of a small size, and of a spheroidal or ellipsoidal form. Their fracture is crystalline, and they are often perfectly transparent, but in all cases where they are in immediate contact with the trap they possess no external crystallized form. It is only when in contact with quartz or agate that they exhibit an external as well as an internal appearance of crystallization. ”

“ The character of the amygdaloid varies considerably according to the substances which it contains, and these are green earth, calcareous spar, quartz, and chalcedony.
Green earth or chlorite, is the substance which perhaps occurs most frequently, and it is indeed that by which the trap of Kinnoul is particularly distinguished from almost all the rocks of this tribe in Scotland. The nodules vary but little in size, and although they are occasionally met with of the bulk of a filbert, they are generally little larger than a grain of mustard. ”

“ In many parts innumerable detached fragments may be seen imbedded in the trap, the whole of the schist displaying a greater disorder and confusion than the sandstone does in any of the instances hitherto described. In a few places another singularity may be remarked. Veins of different dimensions, and ramifying in different directions, are to be found traversing considerable portions of the rock, and in some instances terminating in the schist, with which they are continuous. ”

“ The last and most remarkable circumstance occurring in Kinnoul is that of an extensive range of those junctions with other rocks which are supposed by many persons to demonstrate the igneous origin of trap.
About the middle of the broken face of the hill a portion of a stratum of sandstone is to be observed entangled in the mass of the rock, and at the same time much broken and bent. ”

“ The stalactitical forms which line the hollow cavities, can perhaps have resulted only from watery infiltration, yet the existence of the previous cavities can scarcely be accounted for by that hypothesis which considers trap as a deposit crystallized from an aqueous solution of earths.
Different substances are found occupying veins as well as nodules in this rock; among these, calcareous spar and quartz are the most common. Sulphate of barytes occurs more rarely, as do chert and agate, the latter of which often exhibits the same zoned appearance in the veins which it does in the nodules. ”

“ It is incumbent on that hypothesis which explains one of these difficulties, to explain the others also, or at least to require nothing which shall exclude the means of explaining them. It is almost needless to say that I here allude to the different explanations which the aqueous and the igneous theories of the origin of trap have given of the imbedded nodules. Each hypothesis has its difficulties when it refuses to yield ground to its antagonist ”

“ Sometimes the rock is both porphyritic and amygdaloidal, but perhaps the greater part of the whole hill consists of a rock merely amygdaloidal, the base of which is either the well characterized basalt above described, or that variety which is less marked, or else a rock intermediate between this latter and indurated clay. ”

“ The nodules of this description are of various magnitudes and often of a foot or more in diameter, and, like the smaller ones, are shut in and surrounded by the solid rock on all sides. It is more common for the boundary of the quartz in the immediate vicinity of the trap to be formed of various zones of coloured chalcedony. The quartz in this case assumes a peculiar well-known aspect, and is called in the Wernerian nomenclature, amethyst, although most commonly of a white or watery appearance. Different zones of chalcedony and quartz will even at times succeed each other in the same nodule. ”