SCIENCE-GOSSIP. 



281 



GEOLOGY 



CONDUCTED BY EDWARD A. MARTIN, F.G.S. 



Fossil Turtles. — There has been found in the 

 basement-bed of the London Clay at Northwood, a 

 turtle (Lytoloma iovgiceps), measuring seven and 

 a-half inches long and six and a-half inches wide. 

 This is rare in the London area. With the turtle 

 were found Panopaea, Cytherea and Pectunculus. — 

 G. Fletcher Brown, 3, Topsfield Parade, Crouch End, N. 



Immature Ammonites. — The Rev. J. F. Blake, 

 F.G.S., an authority on the subject of fossil 

 cephalopods, has given it as his opinion that many 

 of the small ammonites found in various zones 

 of the Lias, were but the fry of larger ammonites, 

 and scarcely worthy of a name. He did not think, 

 speaking generally, that one in ten of all the 

 ammonites which have ever been collected any- 

 where were full-grown. 



The Vale of Clwyd. — In the " Proceedings 

 of the Liverpool Geological Society," 1897-1898, 

 Mr. G. H. Morton, F.G.S., publishes a paper 

 on " The Carboniferous Limestone of the Vale 

 of Clwyd." The district is classic geological 

 ground, the vale consisting of a tract in North 

 Wales, twenty miles in length, running from 

 the coast at Rhyl toward the south-south- 

 east. It is eight miles wide at the sea, 

 four miles wide at St. Asaph and at Denbigh, 

 two and a-half miles at Ruthin, whilst at 

 Llanfair Dyffryn Clwyd, near the head of the 

 vale, it is little over a mile across. The River 

 Clwyd runs along the valley for about sixteen 

 miles of its length, and is joined by the Elwy near 

 Rhyddlan. Along the upper part of the vale the 

 Triassic strata are frequently uncovered, but are 

 hidden at the northern end by a deep and increas- 

 iing thickness of drift. The Trias is most fre- 

 quently exposed along the margins of the vale, and 

 this formation rises on each side towards the 

 Carboniferous and Silurian rocks, and apparently 

 suffered great denudation along the centre in 

 pre-Glacial times. Except where the valley 

 reaches the sea, it is bounded by hills, formed 

 of Wenlock Shale, but fringed with a band 

 of Carboniferous Limestone, which presents a 

 conspicuous feature in the ground. The Wen- 

 lock Hills rise to as great a height as 1,823 

 feet, whilst the Carboniferous Limestone reaches 

 its greatest height of 866 feet at Moel Hiraddug. 

 At the head of the Vale, the Limestone reaches a 

 height of 661 feet above ordnance datum, whence 

 a magnificent view down to the sea is obtained. 

 Here, too, a great number of quarries have been 

 opened in the Upper Grey Limestone, and fossils 

 are abundant, large masses of coral being con- 

 spicuous. In the quarry at Faenol a mass of 

 Lithostrotion regium has been exposed for many 

 years. The following species were also observed in 

 the same quarry : Athyris ambigua, Orthis michelini, 

 Productus cora, P. fimbriatus, P. giganteus, P. latis- 

 simus, Spirifera bisulcata, Clisiophyllum bipartititm, 

 Cyathophyllutn murchisoni, Lithostrotion irregulare, L. 



martini, L.portlocki, Phillipsastrea radiata, Syringopora 

 geniculata and Zaphrentis cylindrical. Mr. Morton 

 refers in detail to exposures at various spots up the 

 Vale, and gives analyses of the rock and lists of 

 fossils where necessary. Altogether the paper is- 

 an admirable one. 



Hutton's "Theory of the Earth." — The third 

 volume of this epoch-making work, which is being 

 published by the Geological Society of London, 

 under the editorship of Sir Archibald Geikie, is 

 in the press, and copies can now be ordered of the 

 Secretary of the Society. Three shillings and 

 sixpence is the amount charged for this work to 

 the public. 



Pitchstone. — I am not sure if a more effective 

 rock-section micro-slide than one of pitchstone 

 could be shown to an unsophisticated native or to 

 a casual friend. Now, what is pitchstone, and 

 what do we see when we view down the magic 

 tube a good section of this rock ? It may be 

 described as a vitreous rock with resinous 

 lustre and splintery fracture, or as a porphyritic 

 rock of a trachytic texture with glassy ground- 

 mass. In the British Islands it occurs in Mull, 

 Lamlash, Arran, Rum, Canna, Eigg, in Cornwall, 

 and in counties Down and Antrim. The pitchstone 

 of Arran is of a dark-green colour, about as hard as 

 felspar, and contains about 63 per cent, of silica, 

 13' alumina, 6-2 soda, 4^4 lime, and 3-8 protoxide 

 of iron. It is supposed to be a mechanical mixture 

 arising chiefly from the fusion of quartz and 

 felspar, and its structure, as exhibited under the 

 microscope, is quite unique among British rocks. 

 The principal constituent is seen to be a nearly- 

 colourless glass which is not doubly refractive, 

 and this is richly strewn over with numerous 

 needle-shaped tiny crystals (microlites). The 

 spectacle is decidedly remarkable when the rock 

 section is observed by polarized light with crossed 

 nicols, for we behold a number of brilliantly 

 illuminated needle crystals set off sharply against 

 a back-ground which is quite dark (isotropic). To 

 what mineral species do these tiny crystals belong ? 

 Zirkel took them to be hornblende ; Allport, 

 Vogelsang and Rosenbach thought they were 

 pyroxene ; but later on, Allport again proved that 

 Zirkel's view was right, i.e. they are hornblende, 

 built round, as it were, a central core of glass. 

 In some slides of pitchstone, besides the glass 

 and the microlites, other crystals and fragments 

 are seen, consisting of quartz, felspar, pyroxene, 

 and iron oxide. It is not the picturesque 

 aspect of the cut rock that is alone interesting. 

 Its peculiar texture and structure are of immense 

 suggestiveness. The pasty, amorphous glass and 

 the crystal, well and truly formed, existing side 

 by side, constitute a phenomenon replete with 

 speculative problems. Pitchstone is usually asso- 

 ciated with decidedly plutonic rocks, i.e. rocks 

 which have solidified not at or near the surface, 

 but at a considerable depth below. Moreover, 

 it contains as much as six per cent, of water, 

 whereas ordinary glass does not contain an3'. 

 Hence it has been suspected that the glassy texture 

 of pitchstone is only apparent, and is the conse- 

 quence of transmutation by aqueous agency. How 

 have the crystallites been produced ? Are they 

 simply larger manifestations of a much finer 

 crystalline aggregate composing the whole of the 

 rock ; or are they products of devitrification of an 

 originally glassy ground-mass, and formed subse- 

 quently to its first deposition ? — (Dr.) P. Q. Keegan, 

 Patterdale, Westmorland. 



