Alfred Harker

Camborne Redruth Community Hospital

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II.—On Porphyritic Quartz in Basic Igneous Rocks

Geological Magazine (1892)

Alfred Harker

Although the old-fashioned ideas as to the association of different minerals held by Breithaupt and others have been found to require much modification, there are still certain rules which hold with a high degree of generality. They are, indeed, merely consequences of the principle that the most important of the factors which determine the mineralogical constitution of an igneous rock, is the chemical composition of the magma from which it is formed. Roughly speaking, we may say that original free silica and acid silicates occur characteristically in acid rocks, more basic silicates in basic rocks.

Porphyritic

Igneous petrology

Generality

10.1017/s0016756800195962

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Citations (3)

The Tertiary crust-movements in the Inner Hebrides

Transactions of the Edinburgh Geological Society (1905)

Alfred Harker

The hypothesis of a causal connection between igneous action and displacements of the earth's crust is no new idea in geological speculation. Von Buch's theory of "Craters of Elevation" has indeed passed into limbo, and geologists now regard igneous extrusions and intrusions as the effect rather than the cause of crust-movements, or at least consider the two sets of phenomena as joint-effects of the same causes. From this point of view the subject has been developed by Lossen, Suess, Brögger, Marcel Bertrand, Mrs Ogilvie Gordon, and other writers. The investigation of Skye and the Small Isles, in which I have been engaged for some years on the part of the Geological Survey of Scotland, has led me to believe that the general principle may be followed into somewhat farther detail, and in particular that different manifestations of igneous activity may be related to different types of crust-movements. In trying to decipher the history of Tertiary igneous action in this country, a history comprising many successive episodes, I have found it necessary to distinguish two parallel series of events which were in the broadest sense contemporaneous, but more precisely alternated in some measure with one another. These are, respectively, the regional , which affected the whole area of the "petrographical province," and the local , which were closely connected with certain special foci of activity distributed through that area.1 These two series of igneous rocks seem to be related to two different orders of crust-movements, which may also be distinguished as regional and

10.1144/transed.8.3.344

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Citations (1)

VI.—Berthelot's Principle applied to Magmatic Concentration

Geological Magazine (1893)

Alfred Harker

It is now a recognized fact that many igneous intrusions, especially of basic rocks, grow more basic in composition from the centre to the margin. This type, of what Vogt terms “magmatic concentration,” has been discussed by several geologists, who agree in supposing that in such cases a magma, originally homogeneous, has become differentiated by the migration of the less soluble (more basic) ingredients to the cooler marginal region of the magma-reservoir.

Margin (machine learning)

Igneous differentiation

10.1017/s0016756800191642

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Citations (3)

Some Aspects of Modern Petrology

Science (1911)

Alfred Harker

10.1126/science.34.873.353

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Citations (3)

III.—The Cause of Slaty Cleavage: Compression v. Shearing

Geological Magazine (1885)

Alfred Harker

The received theory of Slaty Cleavage has generally been held to afford a complete explanation of the observed phenomena. The proximate cause of the structure was shown by Dr. Sorby to be a superinduced arrangement of the flat and long-shaped fragments constituting the rock, in virtue of which they tend to lie in, or nearly in, the planes of cleavage; this arrangement being assisted, as Mr. D. Sharpe advocated, by a flattening of those particles themselves. These changes were ascribed to great lateral compression of the rock in the direction perpendicular to the cleavage-planes, together with some expansion along those planes in the line of their dip; and a great mass of evidence was brought forward to support this theory.

Flattening

Cleavage (geology)

Shearing (physics)

10.1017/s0016756800188223

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Citations (9)

Notes on the Igneous Rocks of the English Lake District

Proceedings of the Yorkshire Geological Society (1902)

Alfred Harker

I.—The Ordovician Volcanic Series. This series is divisible stratigraphically and petrographically into several distinct groups, the distribution of which in the district has been indicated by Mr. Marr. * ( a ) Falcon Crag Andesite Group. —These rocks are found chiefly in the country south of Keswick and eastward to the St. John’s Vale. They are pyroxene-andesites, and thin slices show under the microscope sometimes hypersthene (converted to bastite), sometimes augite, sometimes both. To the eye the rocks are pale greenish to dark grey, with a compact ground-mass, usually containing scattered minute porphyritic felspars, or more rarely abundant crystals up to ¼ inch in length. Many of the flows are amygdaloidal. There are rare flows of more basic lavas, besides some tuffs. ( b ) Eycott and Ullswater Basalt Group. —The most widely distributed group of all. The basalts, like the andesites, often contain altered hypersthene; they are sometimes rather rich in magnetite, but olivine does not occur. These rocks are usually darker and denser than the andesites, but a discrimination is not always possible in the field. Part of the rocks are porphyritic, and in some there are conspicuous crystals of felspar (bytownite or labradorite) which on Eycott Hill reach a diameter of an inch or more. Amygdaloidal varieties are found. Basic tuffs are sometimes intercalated among the lavas, and in some places there are andesitic flows included in this group. ( c ) Scawfell Tuff and Breccia Group, with Kentmere-Coniston Slate-Band. —This group presents itself under two different phases. (1) In the central ...

Lake district

10.1144/pygs.14.3.487

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Citations (6)

Geology of the British Isles

P. G. H. Boswell Grenville A. J. Cole Arthur Davies Charles Davison John Evans

Source

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Citations (2)

Petrological Notes on some Boulders from the Boulder-clays of East Yorkshire. (An Appendix to Mr. Lamplugh’s Paper.)

Proceedings of the Yorkshire Geological Society (1889)

Alfred Harker

[NOTE.—The numbers in brackets refer to slides in the Woodwardian Museum Cabinet at Cambridge.] [929] From the Lower Purple Clay, South of Withernsea. A conglomerate including small rounded pebbles, some of dark limestone, about one-fifth of an inch long, and grains of quartz in a compact ground. Under the Microscope a slice shows abundant rounded grains of quartz, with rolled pebbles of (probably dolomitized) limestone, imbedded in a mass of carbonate of lime. This calcareous matrix often shows the characteristic cleavage of crystalline calcite; but in other places are little ellipsoidal concretions with both concentric and radial structures, and showing the black cross in polarized light. Though less coarse than the usual type, there can be no doubt that this rock is the “Brockram,” which forms the basement of the Permians in the Eden Valley. A slide [514] from near Appleby shows precisely the same characters, including the little oolitic concretions. Brockram boulders were long ago recorded by Phillips from the Yorkshire coast, where they are not at all uncommon. [930] From the Basement Clay at Bridlington Quay. A dark grey doleritic rock, showing little glistening felspars in a dark compact ground-mass. Micro. The porphyritic felspars, about one-eighth of an inch long, are rather rounded, as if by corrosion, and sometimes enclose portions of the ground-mass. They are very clear, and show Carls-bad-twinning, but the finer twin-lamellation is very difficult to detect. This is owing to the nearly straight extinction of the crystals, which must belong to a variety ...

Hornfels

Conglomerate

Siltstone

10.1144/pygs.11.2.300

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Citations (1)

Supplementary Notes on the Metamorphic Rocks around the Shap Granite

Quarterly Journal of the Geological Society (1893)

Alfred Harker J. E. Marr

I. Introduction. In a paper presented to this Society two years ago we described at some length the phenomena of metamorphism exhibited by the various volcanic and sedimentary rocks around the Shap Granite. Since that time re-examination of some of the rocks, in the light of what we have seen in other parts of the Lake District, has enabled us to make certain corrections and additions to our work ; and we now offer these with the intention of rendering the description of the metamorphism in some measure more complete. The chief correction we have to make relates to the volcanic rocks on the northern side of the granite, which we formerly classed with the intermediate rocks (andesitic lavas and ashes) seen to the west of the granite. We now find that they constitute a distinct group of more basic composition. To such a conclusion, indeed, we should have been led by the low silica-percentages of some metamorphosed specimens from Low Fell,² a fact which we contented ourselves with recording as inexplicable on the supposition that the rocks had been originally similar to the Stockdale andesites. We have since learnt that basic lavas are very widely distributed over the Lake District, and that the recks on the northern side of the Shap Granite must be placed in this division. They may with propriety be named basalts, although, on account of the absence of olivine, some petrographers would prefer to call them basic andesites. Since our description of the metamorphosed intermediate

Andesites

10.1144/gsl.jgs.1893.049.01-04.53

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Citations (20)

III.—Notes on the Physics of Metamorphism

Geological Magazine (1889)

Alfred Harker

The problem of the metamorphism of rock-masses is one in which increased study has not led to unanimity of opinion. The literature of the recent Geological Congress in London suffices to remind us, how widely divergent are the conclusions to which various geologists have been led by researches in the field. Since, then, the a posteriori line gives such very different results in different hands, it may be worth while to revert for a moment to the deductive method, and try to trace the consequences, in this connexion, of admitted physical principles.

Unanimity

TRACE (psycholinguistics)

10.1017/s0016756800175727

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