The geology of Helvellyn and the southern part of Thirlmere

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Abstract
Since the official examination by the Geological Survey in the seventies of the nineteenth century, more recent work on various portions of the Lake District, carried out by Mr. E. E. Walker, Mr. J. F. N. Green, the late Professor Marr, Dr. G. H. Mitchell, and the present author, has shown that acid rocks are widespread throughout the area and, as foreseen by Sir Archibald Geikie, that their outcrops furnish important facts which any interpretation of the structure of the district, to be convincing, must explain. The original object of the present communication was to describe the development of this type of rock in the neighbourhood of Helvellyn, an area which, since the time of Clifton Ward and J. J. H. Teall, has received only passing and incidental mention. As the work progressed, however, it was found necessary to extend the investigation and to include the mapping and description of the whole of the Borrowdale Volcanic Series as found in this particular area. J. C. Ward, who described the Helvellyn range very thoroughly, fully recognized that an unusual type of rock was present in this locality. He discussed it in detail in his memoir (1876, p. 24), but, led astray by his views as to its metamorphic origin, he failed to map it in detail. He made no attempt to separate these flows of acid lava, but included them with other rocks on the one-inch map, indicating them by a symbol representing “mostly altered ash”. This vital omission excludes from