230 Re]3orts and Proceedings — Geological Society of London. 



Ulrich & Schofield, J7. ? gracillima, Salt., H. civgulata, His., and 

 IB. articulata, Sow. The species of Ectomaria are all derived from 

 the Cambrian and Ordovician rocks of Scotland, and the species of 

 Bormotoma from various beds, ranging from the Durness Limestone 

 to the Upper Ludlow rocks. An account of the distribution of the 

 genera over Europe and America is also given. 



IL— March 8, 1899.— 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 Bhynchonella Cuvieri to that of 

 Micraster cor-anguinum." By Dr. A. W. Eowe, F.Gr.S. 



The author has endeavoured to show, by means of rigid zonal 

 collecting on a large scale, from the White Chalk of the Southern 

 and South-Eastern coast-sections of England, that the genus Micraster 

 is one and the same form gradually evolving from the more simple 

 to the more complex. In doing this, he also contends that the genus 

 may be divided into definite groups, each or several of which are 

 absolutely diagnostic of the various Chalk zones, as defined by 

 Barrois. The conclusions arrived at point to the regular and con- 

 tinuous deposition of the White Chalk, and strikingly confirm the 

 general accuracy of Barrois's zoning. 



The paper gives a minute comparison and description of the 

 genus Micraster from a general point of view, and from that of 

 a group, and deals particularly with the essential details of the test 

 of the especial groups characteristic of each zone. The author claims 

 that, so far as Micraster is concerned, each zone is marked by 

 a definite facies of essential characters of the test, which are purely 

 horizonal, and that all species and varieties, however divergent they 

 may apparently be, occurring at any given horizon, are stamped with 

 the impress of these marked horizonal features. 



The author proves that, while in an isolated instance one may be 

 unable to decide the horizon in the White Chalk whence a specimen 

 of Micraster was derived, in the ninety -nine other cases the 

 diagnostic features described by him point unerringly to the exact 

 horizon, and thus afi'ord a valuable aid to stratigraphical geology, 

 especially as the essential zonal features of the test are easily made 

 out in the field. 



The author discusses the four groups into which the species of 

 Micraster in these zones may be placed, and describes in detail the 

 species in these groups. 



The paper was illustrated by photographs, micro-photographs, and 

 lantern-slides. 



2. "On a Sill and Faulted Inlier in Tideswell Dale (Derbyshire)." 

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



The compact dolerite in the marble quarry in Tideswell Dale has 



been generally described as a lava ; but Sir A. Geikie, in his 



' Ancient Volcanoes of Great Britain," suggested the possibility that 



it might be a sill. in the present paper the author endeavours 



to prove that the rock is really a sill. The compact dolerite rests 



