92 Correspondence — F. R. C. Reed. 



Although ia Great Britain pillow-lavas and cherts are widely 

 distributed, there are few records of iron-ore deposits in connection 

 with them. In other parts of the world, however, some of the most 

 important beds of iron ore belong to this association. In Germany 

 the Lahn and Dill districts and the Upper Harz contain beds of iron 

 ore in Middle and Upper Devonian strata ; these rest on pillow-lavas 

 and tuffs (schaalsteins), and are not uncommonly interbedded with 

 cherts. For several years it has been clearly realized by German 

 geologists that these deposits are products of the pillow-lava 

 eruptions ; and it is considered that they may have originated from 

 emanations of ferric chlorides and other salts arising from the lava- 

 flows during cooling. 



The greatest iron-ore field of the present day is the Lake Superior 

 district of the United States. Iron formations consisting of cherts 

 and beds of haematite (sometimes siderite, magnetite, or limonite) are 

 there found in pre-Cambrian rocks, principally the Keewatin and the 

 Huronian. According to Van Hise and Leith, they are in all cases 

 attended by eruptions of pillow-lava, and the source of the iron is to* 

 be traced to discharges of soluble iron salts proceeding from these 

 lavas during their outflow, or shortly after they had come to rest. 



In Northern Sweden and Lapland great masses of iron ore have 

 long been known, and are mined on a very extensive scale. They are 

 principally magnetite, with varying amounts of apatite, and are 

 usually associated with syenitic eruptives, frequently rich in albite. 

 Recently it has been shown that in the Kiruna district these 

 eruptives belong to a suite in which pillow-lavas are strongly 

 represented, and that all the rocks are very commonly albitized. 

 The " syenites" (also described as " keratophyres ") are the acid or 

 leucocratic members of a " spilitic" suite. The iron-ore deposits are 

 variously interpreted as stratified beds, magmatic segregations, and 

 contact deposits. 



Submarine deposits of iron ore in beds, such as the Noblehouse and 

 Garron Point deposits, resting on or associated with pillow-lavas, are 

 accordingly of worldwide distribution and sometimes of the highest 

 economic importance. Their formation is due to the abundant 

 discharges of vapours and salts from the cooling lava-flows, and, like 

 the albitization and siliceous formations that characterize this group 

 of rocks, they mark the special propensity to pneumatolytic discharges 

 which is one of their distinctive peculiarities. 



CORRESPONDENCE. 



THE YUNNAN CYSTIDEA. 

 Sir, — Dr. Bather's articles (Geol. Mag. November and December, 

 1918) on the Yunnan Cystidea described by me in the Palceontologia 

 Indica, n.s., vol. vi, Mem. No. 3, 1917, appear to require some 

 notice, if only to correct some unfortunate mistakes into which he 

 has fallen. But in the first place he may be congratulated on 

 having at last published a clear terminology and definition of the 

 morphological planes of the Cystidea which will avoid subsequent 

 misinterpretation of his descriptions. 



