THE 
GEOLOGICAL MAGAZINE. 
NEW SERIES, DECADE, V.- ,WOL. IN; 
No. IV.— APRIL, 1907. 
OR GAEINEASE, “Almere aS. 
Wine SOAStYa fe 
I.—A Caste or MrramorpHism oF CHALK. 
By Gro. C. Goven, A.R.C.S., B.Se., F.G.S., 
Professor of Natural History, Roy al Agricultural College, Cirencester. 
(PLATE V.) 
ASES of metamorphosed limestones where the calcium carbonate 
has been converted by contact with some igneous magma into 
a cale-silicate rock are by no means rare, one of the best known being 
the conyersion of the Coniston Limestone of the Lake District by the 
Shap Granite into a rock with various calc-silicate and other minerals 
such as Wollastonite, Omphacite, etc., as described by Messrs. Harker 
and Marr.! But as far as I know, no case has been recorded where 
ordinary white chalk has been similarly changed. Dr. Hibsch? 
describes a case of baculite marl containing Foraminifera which has 
been altered by contact with dolerite. The Foraminifera disappear 
and the rock becomes a granular limestone with epidote, forming 
a cale-silicate hornstone. This seems the nearest case to the alteration 
in co. Antrim I am about to describe. 
As most geologists know, the Irish Chalk is considerably harder 
than the English variety, due, it is usually considered, to the baking 
action of the overlying basalts, although this idea is far from being 
generally accepted. In places it is not uncommon for dykes to have 
penetrated the chalk and to have altered it at the pomt of contact to 
erystalline limestone. Nearly a century ago Berger® stated that 
“the chalk is frequently traversed by basaltic dykes, and often 
undergoes a remarkable alteration at the point of contact. The 
change sometimes affects 10 to 12 feet from wall of dyke, gradually 
decreasing from the dyke. The extreme effect presents a dark-brown 
crystalline limestone, the crystals running in flakes as large as those 
of coarse primitive limestone, etc.” Other instances of the dis- 
appearance of the organisms and the production of crystals of calcite 
could be quoted. 
In 1904 Miss M. K. Andrews, of Belfast, while examining the 
V QEGeS2, 189 
2 Verhandlungen Geol. Reichsanstalt, 1889, p. 204. 
3 Trans. G. §., vol. ii. 
DECADE V.—VOL. IV.—NO. Iv. 10 
