160 Geological Society : — 



February 22nd.— W. Wbitaker, B.A., F.R.S., President, in the Chair. 

 The following communications were read: — 



1. ' On Varieties of Serpentine and Associated Rocks in Anglesey.' 

 By Prof. T. G. Bonney, D.Sc., LL.D., F.R.S., F.G.S., and Miss C. 

 A. Raisin, D.Sc. 



The paper adds certain details to the description published in 

 1881 by Prof. Bonney, and notices some important outcrops near 

 Llyn Dinam and Llyn Penrhyn. 



In the serpentine, enstatite frequently, and diallage some- 

 times, are present : tbese crystals being occasionally lustre-mottled. 

 The most remarkable variety (which occurs distinctly at seven 

 localities, although generally over a surface of only a few square 

 feet) is that termed 'variolitic serpentine.' On a weathered surface 

 knobs project as upon a variolitic diabase : these, under the micro- 

 scope, are found to consist mainly of fibrous aggregates, probably 

 actinolitic, sometimes exhibiting a radial structure but rarely a 

 concentric one. A carbonate is also present in variable amounts. 

 The authors think that probably the original magma of this rock 

 was differentiated, one part representing a pyroxenic constituent 

 and becoming modified by alteration into the spherules ; the other 

 (olivine) producing ordinary serpentine. 



Intrusive in the serpentine are diallage-rock, enstatite-rock, and 

 rocks composed of both these minerals, all usually containing some 

 serpentine. Various actinolitic rocks, often truly ' actinolitic schists,' 

 probably have been derived through crushing from a diallage-rock 

 (or one allied to it). Other intrusive rocks are the gabbro, which 

 forms large, important masses, a dyke of enstatite-gabbro, a por- 

 phyrite (?), and some greenstones. 



A secondary development of tufted actinolite (compared by the 

 authors with the actinolite in true schists, and with that in the 

 Bastogne rock) is described from at least four localities. It is 

 probably an indirect result of crushing in rocks which, though 

 petrographically different, all originally contained a pyroxenic 

 mineral which has been reconstituted. 



Further details are given of the talc-schist and the chlorite- 

 schist in this district, confirming the views that the former rock is 

 a modification of a serpentine, and that the latter was originally 

 intrusive. 



The age of these igneous rocks cannot be determined ; but they 

 must be earlier than the great earth-movements to which the 

 pressure-structures are due. 



2. ' Remarks on the Genera Ectomaria, Koken, and Hormoloma, 

 Salter ; with Descriptions of the British Species.' By Miss J. Donald. 



March 8th.— W. Whitaker, B.A., F.R.S., President, in the Chair. 

 The following communications were read : — 



1. 'An Analysis of the Genus Micraster, as determined by rigid 

 Zonal Collecting, from the Zone of Rhynchonella Cuvieri to that of 

 Micraster cor-anguinum? By Dr. A. W. Rowe, F.G.S. 



2. ' On a Sill and Faulted Inlier inTideswell Dale (Derbyshire).' 

 By H. H. Arnold-Bemrose, Esq., M.A., F.G.S. 



The compact dolerite in the marble-quarry in Tideswell Dale has 



