RECORD: Darwin, C. R. Geological diary: (Falkland Islands, in comparison with Henslow's account of Anglesea). [nd] CUL-DAR33.217-222 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed from the microfilm by Kees Rookmaaker, corrections and editing against the manuscript by John van Wyhe 7.2010. RN1
NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Marginal notes are here integrated into the text.
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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.
In the admirable description of the Geology of Anglesea by Prof. Henslow.1 there are some facts. respecting the lower formations which singularly agree with, whilst others are as strongly opposed, to, the foregoing account of the structure of the Falkland Isds.
The stratified mass which occurs lowest in the formations
of Anglesea, is composed of quartz rock, which has
been reduced to a
existed in a pasty condition & whilst in that state. has been modeled
into very singular curvatures. This granulo-crystalline
rock in parts contains "few minute white specks of earthy felspar"
& sometimes a little mica. As seen It seems to pass insensibly
into an overlying chlorite schist. & this latter rock again passes into
an ordinary clay-slate: hence the chlorite schist plays the part of the intermediate
rocks of the Falklands. — Inspection of the coloured map, shows that a NE
& SW line is common to the form of the land, the line of river courses,
& the divisions of the different formations.
cleavage & stratif [lan]dip less
it is also
said "the original bearing" of the
1 John Stevens Henslow (1796-1861), professor of botany at Cambridge University and Darwin's mentor and friend. Henslow 1822, p. 362.
existed in a] pencil insertion.
cleavage & stratif [lan]dip less] added pencil in margin.
This same NE & SW line of bearing appears likewise to be common to the laminated structure. The quartz rock is divided, by a very singular system of cleavage, the laminae being caused by the presence of scales of mica directed in a particular position. The dip is to the N of W. —
In the Falklands, although such a structure is not developed, it may be remembered, where the specks of imperfect mica occurred, there was an obscure tendency to a fissile character. The quartz has been affected by most complicated lines of curvature, which (I suppose) are directed nearly in the line of cleavage. —
The chlorite schist, which succeeds the quartz, has a laminated
structure, which Prof: Henslow P. 371 thinks follows the plane of true stratification
& consequently. Above the chlorite schist, we have
clay-slates of the greywacke series, mingled with some sandstones; in this
formation there occurs again a strongly developed fissile structure, which
is always inclined at a high angle. The position
of certain patches of breccia, appears P 380 "to shew that the laminae do not mark any order of superposition." These laminae seem commonly directed as those of the quartz namely to the E of N.. The absence of a fissile structure, distinct from stratification in the chlorite schist, is a parallel case to the amorphous forms of the intermediate rocks, which in the Falklands are surmounted by laminated clay-slate. In the Old red sandstone (which in places
seem insensibly passes into the grewwacke), P 389 "the strata generally bear in the same direction as the laminar tendency of the last formation, but their average dip is not so considerable."
How exactly does this correspond to what happens in the Falklands; although there the relative superposition is reversed the less inclined strata being below instead of above the nearly vertical laminae! —
In the same formation Prof. Henslow says, P 389 "In several small quarries about Lleckynfarwy, we meet with a laminar tendency, often thin slaty, inclined at an angle of 65°
towards a point 30° the W of N. This direction of dip prevails throughout the remainder of the district." — Reasons are afterwards assigned "for supposing that these laminae to be wholly independent of the original order of depositions." P. 390 Here then we have occasionally appearing in the midst of coarse materials, which are regularly stratified, patches of finer sedimentary rocks with a laminated structure. The overlying coal measures strike in the line, so often mentioned, but the angle of inclination varies much. There is one most curious phenomenon with respect to a grit of this formation which P 395 "is composed of small angular fragments of quartz studded with white earthy specks of carbonate of lime. These specks are frequently arranged in parallel lines inclined to the direction of the strata: an effort, if so it may be called, to produce a fissile texture in a coarse substance, where it could scarcely have been expected. The particles of the quartzose fragments appear likewise to have undergone a rearrangement: for several contiguous fragments possess a common cleavage." —
In conclusion it appears to me certain that the lines of elevation
are parallel to the lines of cleavage, as was
supposed believed to
be the case in the Falklands.
I do not clearly see the reasons.
In the case of the quartz. Prof: Henslow thinks the cleavage is owing to an action subsequent to the flexure of the strata & therefore to the softened state of those rocks. Such cleavage might as well be impressed on the quartz. at any subsequent period, as that the particles, in the far more modern formation of the grit, should be arranged by the same or a closely allied power. If we believed the mica, which determines the cleavage in the quartz, to be a mineral produced in that rock, when in its softened state, this view becomes the more probable. — However this may be, can we suppose otherwise, than that the laminated
tendency structure in the grawwacke & old red sandstone was the result of an action, which took place at successive periods, & prior to the consolidation of those rocks from aqueous deposition. —
appears to think remarks P 365 "that
the present structure (of the quartz rock) may suggest an idea, that crystalline
I do not clearly see the reasons.] pencil in margin.
assisted by moisture & pressure, is an agent of sufficient power to have produced the similar but still more perfect texture of the oldest stratified rocks." Is not the metamorphic action of heat a more simple method of accounting for the phenomenon? Although the curved strata call to mind the manner of elevation in the Falklands, there is this wide difference, that in Anglesea the solid rock has been frequently propelled upwards, causing unconformable junctions between the various formations. I look at these facts, as most valuable confirmations of the opinion, that in certain districts, a tendency towards a laminated structure in a particular direction, remains constant, throughout successive deposits of different epochs: and that the lines of elevations have also a tendency to follow the bearing of the preexistent cleavage. —
At Falklands. the symmetry of thread like fissure filled with quartz go to prove the elevation of cones of quartz a simultaneous action.
Bougainville Voyage Tom I1 talks of great bones some way in shore. ? is it worth reference. —
M. Lesson p. 199.2 describes geology of Falkland
V. R.N. p. 102
Murchison no aspect of Falkland rock R.N 1523
Falkland no erected boulder ∴ subsidence.
Bougainville saw bones of whales far inland — which I believe is a lie —
؟ Proofs of elevation?
؟ recent shells?
1 Bougainville 1772, vol. 1.
3 Murchison Red notebook, p. 58.
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