ORIGINAL ARTICLES. 



Beerbacliite at the Lizard. 



By Professor T. G. Bonney, Sc.D., F.R.S. 



TN my paper on the Lizard this rock is mentioned, but without 

 -*- a specific name, for I had not then seen the figure given in 

 1898 by Rosenbusch {Elemente der GesteinsleJire, 1898, p. 51). He calls 

 it Beerbachite, and describes it on p. 219 as one of his Ganggesteine, 

 among those von malchitischem Habitus, quoting an analysis. The 

 name appears to have been given by Chelius to a rock from Franken- 

 stein in the northern Odenwald, where it forms small veins in a 

 gabbro, and has the following aimlysis : SiOs = 47-21, AlgOg == 20-52, 

 FcoOs = 7-48, FeO = 5-32, MgO = 4-16, CaO = 8-63, NaaO = 

 5-17, K2O=0-33, H2O=0-34, PA = O"^^ with 0-19 FeS^ and 

 O'lO hygroscopic water. He states that the rock is fine-grained to 

 compact, not lustrous, grey to pale grey in colour, consisting of a 

 panidiomorphic mixture of labradorite and diallage, with a variable 

 amount of hyperstherie and magnetite, and that olivine-bearing 

 varieties occur. In these commonly a brown hornblende replaces 

 the diallage, in which the above-named constituents are poecilitic. 

 He states that beerbachite also occurs in gabbro near Harzburg and 

 in the Isle of Mull. 



At the Lizard the best outcrops of the rock are near Luggar 

 Cove, to the south of Porthoustock, and I call it in my diary for 

 July 6, 1894, when I j^articularly studied that part of the coast, 

 " the brown-speckled rock," but it occurs, sometimes in considerable 

 force, from near St. Keverne to the neighbourhood of Porthallow,^ 

 and I observed and collected it, a few years earlier, from the crags 

 north of the Balk, where, however, it is less well exposed and rather 

 more weathered . The f oUov/ing shortened extract from my diary for 

 1894 (which is illustrated by sketches) may serve to show the curious 

 mixture of rocks near the shore to the north of Crousa Down. In 

 certain places the gabbro itself varies much, a coarse variety, with 

 diallage crystals sometimes a couple of inchesin diameter, occasionally 

 appearing in irregular patches and even in little tongues in the finer 

 gabbro. The " brown-speckled " rock breaks into the gabbro, 

 including fragments, patches, and streaks of it, a few inches or even 

 more in length and perhaps one to three in diameter, which often 

 assume a foliated structure. The speckled rock also not seldom 

 appears to be " porphyritic ". The crystals, which may run to about 



^ I studied the coast north of Crousa Down, from St. Keverne, in 1890, with 

 my friends General McMahon and Canon Hill, and again in 1894 with the 

 latter. I revisited it in 1908, when staying with Dr. Flett at Kennack Cove, 

 to discuss sundry points on which we were not agreed, but do not remember 

 that we used then the word beerbachite ; and have since seen a little of the 

 neighbourhood in 1909 and 1912. 



