XXXVIil INTRODUCTION. 
beds during intervals of repose: and so far they are analogous to the fine beds of volcanic 
silt as often formed by the waters of a lake out of the ashes of a modern crater. 
“In the Cumbrian mountains no organic remains are found among these rocks. The 
aqueous deposits seem to have been too often interrupted by igneous action to permit the 
growth of shell-beds and coral-banks. Shells and corals are however found (though rarely) 
among the slate-rocks of Snowdonia: but there the igneous beds are less abundant, and 
were probably poured out at longer intervals of time.” 
These quotations are from a letter to my late honoured friend Wordsworth, the great 
poet of the lakes. They were written in the Spring of 1842, before I had revisited North 
Wales after my first survey of it in 1831 and 1832. I do not wish to change one word of 
them, and I have little to add to them*. 
It might, perhaps, be said that views of a like kind had been published before 1842, by 
Sir H. de la Béche and Sir R. I. Murchison, and that I had eked out these descriptions by 
borrowing from the property of others: and after some words which were let fall at the last 
meeting of the British Association (in 1854) I looked back with some anxiety, among the 
early published Proceedings of the Geological Society of London, to see whether there were 
any printed records of my views respecting the Cumbrian plutonic rocks; and I rejoiced to 
find in the first Volume (p. 400) an abstract of a paper by myself (of the date of May 16, 
1832) containing the following abridged description of the great central group of the “Cum- 
brian Mountains.” 
“This great group, which occupies all the highest and most rugged mountains of the 
region described in the memoir, is essentially composed of great tabular masses, composed 
of different modifications of porphyritic and felspathic rocks, and of quartzose and chloritic 
slate, all the finer portions being derived from a cleavage transverse to the stratification of 
the beds. The modifications of the slate are first described, and it is shewn that they pass, 
on one hand, into compact felspathic slate sometimes porphyritic; on the other, into coarse 
granular and concretionary slaty masses, and through them into breccias, or pseudo-breccias ; 
all these changes being effected without any change of strike or dip. In like manner it is 
shewn that the amorphous, and even semicolumnar, prismatic, porphyries are not only 
arranged in directions parallel to the tabular masses of green roofing slate; but pass them- 
selves into a slaty texture with a strike and dip parallel to those of the true roofing slates. 
They also pass into brecciated masses similar to those which form a part of the slate-groups. 
From these facts,—as well as from the negative facts, that the porphyries never penetrate 
the roofing slate in the form of dykes, and produce no mineral change in the limestone beds 
resting on them,—it is inferred that the whole group is of one formation, which has origin- 
ated in the simultaneous action of aqueous and igneous causes long continued.” 
After the year 1824 I regularly taught the conclusions implied in these quotations; and 
I maintained that many rocks, which now have a perfect porphyritic structure, were so far 
* Appendix to a Guide to the Lakes. John Hudson, Kendal. 
