Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/319

 and conglomerated one. The island of Arran indeed produces some porphyry which appears to possess a structure analogous to this, inasmuch as grains of quartz apparently rounded, not crystallized, are found imbedded in a common base with glassy and earthy felspar. But that structure might admit of dispute.

Such are the most remarkable of the independent facts which I have noticed in this tract of country. Let us now, rejecting all the minuter features which accompany this succession of rocks, extend our views over a wider range, and attempt to investigate their general bearing.

The most continuous bed which has occurred in this view is the limestone. We have seen that it is regularly bedded, without any material disturbance, with an elevation somewhat varying and consequently in an undulating plane, and that it is continuous over a very large tract of country. We have also seen that its elevated edge is prolonged in a line which is straight for a great distance, and which perhaps may prove to be so even through a greater space, when an accurate survey of Scotland shall have been produced. It is therefore the most regular rock of the series now under examination, and may consequently be taken as the point of comparison for the others and as that from which the true relations of the rocks on each side of it must be investigated.

It is indifferent to the object which I have at this moment in view, whether the beds of limestone are considered as having been originally deposited in their present position, or whether by subsequent changes they have been diverted from one more horizontal. It will not be denied that they are deposited rocks.

If we now trace upwards from this continuous bed of limestone awe shall find that it is followed by a large and continuous bed of