Four Theories of the Age and Origin of the Dartmoor Granites have been lately current, viz.:—

(1) The ordinary Plutonic of Sir H. de la Beche.

(2) The Laccolitic-Plutonic of Mr. W. A. B. Ussher.

(3) The Volcanic of Mr. K. N. Worth.

The above all assume the exclusively post-Carboniferous age of the granite, owing to its intrusion into the adjacent Carboniferous rocks;one of the best attested facts in geology.

(4) The combined pre-Devonian non-intrusive and post-Carboniferous intrusive (both plutonic), advanced by myselfin1889.

Mr. Ussher has recently withdrawn his provisional laccolitic hypothesis, on stratigraphical grounds,infavour of one which is practically equivalent to No. 4.

As it is impossible to compress the work of some four years into a magazine article, the present paper must be confined to a sketch of my main argument.

My friend Mr. Worth has, with characteristic generosity, lent me the diagrammatic woodcut illustrating bis ideal volcano;accompanied by the following remark:“Please do not limit your views of me;you owe it to yourself to make your case as strong as possible, and we shall not differ outside our theories.”A too generous opponent indeed, and one hard to oppose.

I undertook this description of certain fossils from the Lower Palaeozoic rocks ofYorkshire at the request of Mr. Marr. The late Mr. Thomas Roberts had begun the piece of work, but left it unfinished. Nearly all the fossils referred to were collected by a party of Cambridge geologists, under the guidance of Prof. Hughes, in the Summer of 1889. The fossils come from two horizons, viz.:—

1. The Phacops elegans zone≍Stockdale Shale series (Valentian) of Wharfe.

2. The Bala beds≍:Coniston Limestone series of Norber Brow and Wharfe.

I was not present when Dr. Callaway's last paper on the Malvern Rocks was read before the Geological Society, and had not the Opportunity of joining in the discussion that followed the reading of the paper.The author has now given his views regarding the “Conversion of Chlorite into Biotite;” in a condensed form in the GEOLOGICAL MAGAZINE (Dec. 1893, p. 535), and has expressly invited criticism on them; I venture, therefore,inresponse to this invitation, to give expression to a difficulty that presents itself to my mind.

It is not necessary to enter into a discussion of the authorities quoted by Dr. Callaway; but I remark in passing that two of the papers referred to prefess to deal with cases of contact metamorphism, and consequently the conditions that must have prevailedinthose cases are altogether different from those which govern Dr. Callaway's supposed case of dynamo-metamorphism.incases of contact action one can readily understand how aqueous acid vapours, or liquids, emanating from the molten igneous rock under high pressure penetrated the adjoining rocks, and carried with them in solution some of the constituents of the igneous magma.

Much light has been thrown of late years on the history of gneiss and other foliated rocks. Geological literature is beginning to bristle with such terms as regional–metamorphism, pressure–metamorphism, dynamo–metamorphism, and the like. Yet longer and less intelligible names will probably follow, which, under the guise of precision, will foster confusion. So it may be a little help to students to set down, as far as possible in plain English, the fruits of some years' study of gneisses and allied rocks.inthis work I have taken nothing on trust, and have tested every inference to the best of my ability. Probably there is nothing original in the results, but they are all the outcome of personal observation, for I have always preferred questioning Nature to reading books. So,inorder to economize time in searching for what has been already said, and to save studding the page with references, I will assure the reader that he is quite at liberty to suppose that “everything has been said by somebody, somewhere.”

Let us begin our observations with a rock which is a fairly coarse granite, occurring in a region affected by great earth-movements, such as mountain-making, which, however, for some reason or wither, has escaped with its structure practically unmodified. Such a rock sometimes may be locally indistinguishable from an ordinary granite, but very commonly a small difference is perceptible, especially on a slightly weathered surface. This has a rather fragmental aspect, the quartz and the felspar presenting a superficial resemblance to unrolled clastic grains,—asinan arkose, the materials of which have been transported only for a short distance. This resemblance does not disappear on microscopic examination. The rock,inshort, has been fractured but not crushed. This structure is exhibited (not to mention other examples) locally in the granite at the entrance of the Yal Rosegg (Pontresina), in the granitoid rock of Twt Hill(Carnarvon), and of Llanfaelog (Anglesey); occasionally also in the so-called Dimetian of St. Davids. This, then, may be regarded as the first stage in pressure-modification.

The Woodwardian Museum has for many years contained a fine series of Skiddaw Slatefossils.in1890 this was increased by the addition of many specimens obtained by themembers of Prof. Hughes' Long Vacation party, and immediately afterwards the University acquired, by donation, the valuable collection of the late Mr. Kiusey Dover. Prof. Nicholson has also presented a large number of Graptolites. When time permits, I hope to be able to describe our unrivalled collection of Skiddaw Slate fossils; mean-while, a few notes on the distribution of these organisms will throw such light on the development of the Skiddaw Slates as is required for the elucidation of the detailed structure of the district, a task which Mr. Harker and I shall attempt to perform ere long.