INTRODUCTION. XXXVil 
year 1822, to examine the district of the northern lakes, and the structure of the Cumbrian 
mountains, “I believed in the igneous origin of basaltic and porphyritic rocks: but I was 
staggered in my creed, and filled with astonishment, almost at every step, when I saw the 
alternating masses of slate and porphyry, and the way in which they were blended together. 
The Wernerian hypothesis has now passed away, and has been extinguished by the more 
mature discoveries of an advancing science; but it lent itself readily to the explanation of 
many perplexing facts; it had the merit, at first sight, of great simplicity; and I may venture 
to affirm, that no one is prepared to understand it, or to do any justice to its author, who 
has not studied, in the field, such phenomena as are continually offered by the Cumbrian 
slates.” 
“The central division of the Cumbrian slate-rocks forms a vast group, that rises into the 
highest and most rugged mountains of the whole region. It contains two distinct classes of 
rock—aqueous and igneous: but they are piled one upon another in tabular masses of such 
regularity, and are so interlaced and blended, that we are compelled to regard them as the 
effects of two distinct causes, acting simultaneously during a long geological period. The 
igneous portions present almost every variety of felstone and felstone porphyry* ; sometimes 
passing into greenstone, and rarely into masses with a structure like that of basalt. All the 
aqueous rocks have more or less a slaty structure, and pass in their most perfect form into 
the finest roofing slates. 
“But why are rocks, so different both in appearance and origin, to be confounded in one 
formation ?—Because nature has made them inseparable. The tabular masses of true erupted 
‘plutonic rock’ alternate with, and pass by insensible gradations into, great beds of breccia 
and ‘plutonic silt.’ The breccias are often as hard as the parent rocks; being cemented by 
a felspathic paste, occasionally studded with garnets and crystals of felspar; and they some- 
times put on a columnar form: and the plutonic silt passes into a hard, flaky, shining rock, 
which often has a transverse cleavage with an uneven, shining, wavy surface (exactly like that 
of some varieties of the German schaalstein). We have only to follow such changes a little 
farther, and we are conducted, without seeing where we pass their boundaries, into great 
deposits of the most perfect roofing slates. Of these slates, quartz in the finest state of 
comminution, and earthy chlorite partly derived from the plutonic silt, are the chief con- 
stituents. 
“The plutonic rocks were poured out under a deep sea; and the breccias were formed 
mechanically (like volcanic breccias found among streams of modern lava), and were cemented 
under great pressure. The plutonic silts have an intermediate structure; but their beds must 
have been spread out by the waters of the sea. The roofing slates are but the extreme case 
of fine aqueous sediment, chiefly derived from the erupted matter, and sinking into successive 
* IT adopted the word félstone from the German miners of Nassau; who by the word féldstein, sometimes describe 
those minerals which we have commonly, but inaccurately, called compact felspar. The words, compact spar, surely involve 
a verbal contradiction, What is described, in the above quotation, as plutonic silt, I now generally call trap-shale— 
a recomposed plutonic rock not supposed to have been ever sub-aerial, and therefore not a true volcanic ash. 
