Charles Lyell made a geological excursion to the Eifel region in Germany in July 1831. He went to examine volcanic rocks and volcanic landscapes. He discussed this outing with Mary Somerville and Samuel & Charlotte Hibbert. It is possible that he observed loess in the Eifel. It is hoped that his Eifel notebook is with the Lyell papers at Kinnordy and that it may be transcribed and published. Lyell spread the word on loess; Von Leonard invented it and Horner enthused about it but Lyell disseminated the essential idea of loess. There is (so far) no clear evidence that Lyell saw and appreciated loess in the Eifel region in 1831. This suggests that his first real encounter with the loess (ground or concept) was in the discussions with the Hibberts in September 1831. He certainly had substantial (reported) encounters in 1832, and was definitely interested by the time of the publication of the Principles of Geology vol. 3 in 1833.
Bunbury, F.H. & Lyell, K.M., 1891. Memorials of Sir C.J.F. Bunbury, 9 volumes. Mildenhall, reprinted by Cambridge University Press.
Hibbert, S., 1832. History of the extinct volcanos of the basin of Neuwied, on the Lower Rhine. W & D Laing, Edinburgh, 262 pp. (section on loess in Loess Letter 67, www.loessletter.msu.edu)
Lutz, H. & Lorenz, V., 2013. Early volcanological research in the Vulkaneifel, Germany, the classic region of maar-diatreme volcanoes. Bulletin Volcanologique 75, 720-743.
Lyell, C., 1833. Principles of Geology v.3. John Murray, London, 297 pp. (reprint Chicago University Press).
Lyell, K.M., 1881. Life, Letters and Journals of Sir Charles Lyell v.1. John Murray, London, 475 pp. (reprinted Elibron Classics 2007).
Lyell, K.M., 1890. Memoir of Leonard Horner FRS FGS v.1. Womens Printing Society Ltd, London, 376 pp. (reprinted Cambridge University Press 2011).
Scrope, G.P., 1827. Memoir on the Geology of Central France: including the volcanic formations of Auvergne, the Velay, and the Vivarais. Longman, Rees, Orme, Brown & Green, London, 182 pp.
Somerville, M., 1831. Mechanism of the Heavens. John Murray, London, 621 pp. (reprinted Forgotten Books 2012).
Wilson, L.G., 1972. Charles Lyell, The Years to 1841: the Revolution in Geology. Yale University Press, New Haven, 553 pp.