Geology of Anglesey by John Stevens Henslow 1822

1 Preface The geological work reprinted in this booklet was far ahead of its time. In 1822, while many geologists were still not fully convinced of the igneous origin of granite, Henslow had already grasped the nature of thermal metamorphism and knew that cleavage and schistosity in sedimentary rocks were independent of the original bedding, being the later products of temperature and pressure. Henslow is best known today for having been the mentor of Charles Darwin. In this early work, published when he was just 26, his independent genius can be glimpsed. The title page of the Anglesey Memoir as published in book form Lord Dinorben’s Library Plate 1 2 published in the Transactions of the Cambridge Philosophical Society, showing that the decision to produce the work as a book was taken by Henslow himself, at Cambridge, in the same year that the Memoir appeared. The Fanning-Evans connection is proved by an autograph MS slip kept along with the volume itself, in which that important local businessman and geologist corrects a mistake of detail in Henslow’s narrative (see figure). The significance of Henslow’s work will be considered below; it is, however, perhaps appropriate to record here the opinion of the eminent geologist and cartographic historian F. J. North 4:- Henslow’s autograph inscription In Henslow’s map the main rock divisions had been recognised and their outcrops delineated, and the sections accompanying it indicate that he had appreciated, in a manner really remarkable for his period, the Little is recorded as to the origin of his plan to survey Anglesey.1 It seems to have main structural features of the island. The errors - imperfections would be perhaps a better word - are such been part of a joint scheme with that other great geological pioneer Adam Sedgwick, the as increase one’s admiration of his work, because they indicate the recognition of certain rock groups, the true nature of which has only been made clear during the present century. The dykes of dolerite (Trap founder of the Cambrian system, to study island geology generally. Neither is anything yet dykes) that can be seen to penetrate the rocks along the Menai Straits are depicted, and the paper includes known of Henslow’s methods nor his itinerary, although it seems likely that he worked also a realistic sketch of the contorted strata of the cliffs opposite the South Stack Lighthouse at Holyhead. from horseback and stayed at the houses of the local gentry. The great bulk of the work Although this appears to be Henslow’s only contribution, it entitles him to be regarded as one of the first of was accomplished in one or at most two seasons around 1821. the pioneers in Welsh Geology. The present writer had the great good fortune to be able to study a rare original copy of Henslow’s Memoir, still in private hands. It bears the bookplate of Lord Dinorben and is inscribed in Henslow’s own hand (see illustration). It is likely, from its known associations, to have been formerly in the Amlwch area, almost certainly the property of Mr Thomas Fanning-Evans2, Manager of the Parys Mountain Copper Mines in the mid - 19th century. It is tempting to suppose that it might have been presented by Henslow either to William Lewis Hughes, the future Lord Dinorben or perhaps to the Williams family of Llys Dulas, (as Dinorben’s second wife was the heiress of that estate) as a token of appreciation for assistance or hospitality afforded him during his Anglesey survey. 3 However, no firm evidence is available. The book is well-bound in a half-calf style, with the binder’s imprint Enoch Jones Binder and Printer, Beaumaris. The title page differs from that of the version originally 1 Since this booklet was completed, there has appeared an excellent full-scale biography of Henslow entitled Darwin=s Mentor by S. M. Walters and E. A. Stow, Cambridge 2001. The work of these scholars, while Fanning-Evans’s autograph correction-slip completely superseding such biographical details as have been included in this booklet, adds comparatively little to our knowledge of the Anglesey survey, except for the interesting possibility that Henslow may have been accompanied and assisted in his work by a number of his Cambridge students., including F. Calvert. See pp. 29- Acknowledgments 31 of their book for these views. 2 Thomas Fanning-Evans Senior 1837 - 1891 was a considerable geologist and published The Mines of the Parys The permission of the Cambridge Philosophical Society to republish Henslow’s Mountain in Trans. Manchester Geol. Soc. 1877 Vol. xiv. pp. 357 - 395. He was the son of Evan Evans 1799- Memoir is gratefully acknowledged. This famous Society was founded jointly by Henslow 1863 who married Mary, daughter of Thomas Fanning of Park Lodge, Llanbadrig, a Dublin barrister. Thomas Fanning-Evans married Mary Elizabeth Morris, daughter of David Morris of Llannerch-y-medd, who was related himself and Adam Sedgwick in 1819. The Memoir appeared in the first volume of its to the Anglesey Morris brothers Lewis, William and Richard. Thomas Fanning-Evans Jnr succeeded his father as Mines Manager and collaborated with Edward Greenly during his survey of Anglesey, around 1910 4 F. J. North Geological Maps their History and Development with special reference to Wales National Museum 3 Colonel William Lewis Hughes was created Lord Dinorben in 1831. See John Rowlands Copper Mountain of Wales Cardiff 1928 p.39; Prof. Douglas A. Bassett had no hesitation in reaffirming North’s view in 1967, in p.65 (note). Lord Dinorben=s seat of Llys Dulas was at Llanwenllwyfo, N..E. Anglesey. The mansion is now his Sourcebook of Geological, Geomorphological and Soil maps for Wales and the Welsh Borders 1800 - 1966 demolished. N.M.W Cardiff 3 4 Proceedings in 1822. The permission of the National Environment Research Council, British Geological Survey, to reproduce the modern outline geological map of Anglesey from British Regional Geology (North Wales) under copyright permit No. IPR/19-11C is also gratefully acknowledged, as is the assistance of Mr and Mrs N. Campbell-Bannerman in the reproduction of Henslow’s hand-coloured map and sections. T. P. T. Williams 2007 Henslow at around the age of 50 5 6 Academic though he was, Henslow was well capable of innovation in the practical field of 2 Introduction instrumentation. Darwin describes in some detail a clinometer designed by him, in Section VI of John Herschel’s Manual of Scientific Enquiry:- John Stevens Henslow It consists of a compass and spirit -level, fitted in a small square box; in the lid there is a brass plate, The Reverend John Stevens Henslow (1796-1861) is remembered today primarily for graduated in a quadrant of 90 degrees, with a little plumb-line to be suspended from a milled head at the his pivotal role in shaping the future career of his pupil and friend Charles Darwin. The apex of the quadrant etc 13 story of Henslow’s intervention with Captain Robert Fitzroy to obtain for the young Darwin the post of naturalist and gentleman’s companion on the Beagle voyage of 1831 - It is tempting to think that Henslow may have given such an instrument to Darwin. 36 is well known, as is his vital ongoing advocacy before the London learned societies of In 1825 Henslow relinquished the Chair of Mineralogy to succeed Rev. Thomas Darwin’s reports home, and his careful handling of the successive returning crates full of Martyn as Professor of Botany. Such was his academic standing that he was elected 5 his protégé’s specimens. unopposed almost immediately on announcing his candidature. From this position of Henslow was in his day a most distinguished naturalist and scholar. Born at Rochester influence he instituted a much needed program of reform in the study of Botany, resulting on February 6th 1796, he was educated first at school in Rochester, then at Camberwell, in the establishment of the first Cambridge Botanic Garden by 1846 and ultimately, by finally matriculating at St. John’s College Cambridge in 1814. He became Professor of 1861, the instigation of a separate Natural Sciences Tripos, awarding degrees in the Mineralogy in his University in 1823 in succession to Professor E. D. Clarke, before he University independent of Theology and Mathematics. He was the author of standard even graduated Master of Arts. He actively pursued fieldwork, making visits to the Isles of works on Botany including A Dictionary of Botanical Terms in 1857. Man and Wight as well as the Anglesey tour which resulted in the paper here reprinted. Henslow held his Chair until his death. As was normal then, he simultaneously held Henslow founded the Cambridge Philosophical Society in November or December religious office14, the Curacy of Little St. Mary=s Church, Cambridge from 1824 to 1832, 6 1819 jointly with Adam Sedgwick, the Woodwardian Professor of Geology. These two and the living of Cholsey-cum-Moulsford in Berkshire from 1832 to 1837. On his lifelong friends seem to have conceived the idea of such a Society on a joint tour in the presentation to the exacting living of Hitcham in Suffolk in 1837, however, he Isle of Wight, with the aim of ‘promoting scientific enquiries and facilitating the increasingly withdrew from Cambridge, ceasing to reside in St. John’s College after communication of facts connected with the advancement of Philosophy and Natural 1839.15 This move was not to the liking of all his scientific colleagues. Charles Lyell in 7 History’. The first volume of its Transactions for 1821 contains, in addition to the present particular wrote to Dr Fleming in unusually strong terms of 8 paper, Sedgwick’s latest work in Cornwall.

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