Alfred Harker — Some Anglesey D//kes. 409 



V. — WOODWARBIAN MuSEUM NoTES : ON SOME ANGLESEY DyKES, I. 



Ey Alfred Harker, M.A., F.G.S., 

 Fellow of St. John's College. 



N the Woodwardian Museum is a collection of nearly a thousand 

 rock- specimens made by Professor Henslow to illustrate his 

 " Geological Description of Anglesea " (1821, Trans. Camb. Phil. 

 Soc. vol. i. pp. 359-452, plates xv.-xxi.). Specimens of the principal 

 dykes were submitted to the examination of Professor Cordier, and 

 his remarks on them are quoted in Henslow's paper. As Coi-dier's 

 determinations date from a time when thin slices of rocks were 

 unknown, and but little attention has since been given to the dykes 

 of Anglesey, it is believed that brief notices of some of the typical 

 rocks from localities easily identified may have an interest for 

 British geologists. As regards the mode of occurrence of the dykes 

 and their effects upon the adjacent strata, little can be added to the 

 accurate descriptions of Henslow, written at a time when the igneous 

 origin of dykes was a proposition to be proved. The specimens are 

 referred to by number in the memoir, and these numbers will be 

 cited below in brackets [ ]. 



We begin with a few specimens froin the numerous dykes exposed 

 on the shores of the Menai Straits. In spite of some interesting 

 variations, these rocks possess many characters in common. They 

 strike usuall}' in directions varying from S.E. to E.S.E., and often 

 coincide with lines of faulting, which are at right angles to the 

 general strike of the strata. The only direct evidence of their date 

 of intrusion is that they sometimes cut through Carboniferous beds ; 

 but if it is allowable to coi'relate these dykes with others having 

 a similar direction in the Anglesey Coal-field, we may infer from Sir 

 A. Kamsay's reasoning that the whole are of pre-Permian age 

 (Geology of North Wales, 2nd ed. p. 264). 



Beginning near Beaumaris, we find in the green contorted schists 

 between Gallows Point and Garth Ferry an enormous number of 

 small dykes of generally compact appearance. The majority of 

 them are not more than a foot or two in breadth, but some attain to 

 about twenty feet. They frequently ramify, but show on the whole 

 a parallel arrangement with an average bearing about S.E. by E. In 

 some cases a certain relative movement of the rocks on opposite sides 

 of a dyke can be verified. Making allowance for the varying sizes 

 of the dykes and the modifications due to more rapid cooling near 

 the edges, the rocks present a general community of megascopic 

 characters; only two of Henslow's specimens have been selected for 

 closer examination. 



Dykes heticeen Gallows Point and Gartli Ferry. 

 [640.] Augite-andesite. — The hand-specimen shows a black, com- 

 pact ground-mass containing numerous clear felspars in the form of 

 parallelograms. These crystals are very flat, and have a marked 

 parallel arrangement, as noticed by Cordier and Henslow. Under 

 the microscope they are seen in cross-sections of elongated shape, 

 0-1 to 0'15 inch in length, and their fluxional arrangement is very 



