558 Reports & Proceedings—Edinburgh Geological Society. 
and it was shown from a purely geometrical standpoint that the 
solution is unique. Their position was also found graphically as the 
intersection in the gnomonic projection of two cubic curves, on which 
any number of points can be obtained by the use of the ruler only. 
W. Campbell Smith : On some minerals from Leadhills. 
Caledonite of pale-blue colour and acicular habit has been 
frequently described in the past as aurichalcite. Hxamination of all 
available specimens of so-called aurichalcite from this locality 
showed that all were caledonite of this acicular habit. The optical 
properties were found to agree with those of caledonite of the normal 
habit. It was shown that in caledonite the plane of the optic axes 
is parallel to (010) and the acute bisectrix is perpendicular to (100), 
and not as stated in Dana and other textbooks. Other remarks 
referred to gold, linarite, minium, and the rare mineral eosite. 
Dr. J. Dragman: An example of porphyry-quartz from the 
Esterel Mountains (France) twinned on the face (1012). 
This twin-law in quartz has previously been observed only by 
Q. Sella in 1858 and has been regarded as doubtful. An example of it 
has been found amongst the porphyritic crystals in the “ blue 
porphyry ” of the Esterel Mountains. A distinction is made between 
the twins of low-temperature rhombohedral q-quartz and those of 
hexagonal £-quartz (stable at a temperature above 575° C.). 
Dr. L. J. Spencer: Biographical notices of mineralogists recently 
deceased ; with an index of those previously published in the 
Mineralogical Magazine. 
EDINBURGH GEOLOGICAL SOCIETY. 
19th October, 1921—Mr. T. Cuthbert Day, F.R.S.H., F.G.8., 
President, in the chair. 
“The History and Development of the Yorkshire, Derbyshire, 
and Nottinghamshire Coalfield.’ By Dr. Walcot Gibson, F.GS., 
Assistant to the Director for Scotland, H.M. Geological Survey. 
The lecturer pointed out that the area beneath which coal under a 
depth of 4,000 feet in considered as proved, amounted to 2,136 square 
miles, containing an actual coal reserve of over 41 thousand millions 
of tons. Many of the valuable seams lie under 3,000 feet depth, 
and there is a considerable area falling within the depth of 2,500 feet. 
Dr. Gibson showed that the sequence was common to all the coal- 
fields of the Midland area, roughly estimated at 8,000 square miles, 
even to the persistence of individual seams of coal and thin beds of 
shale containing a characteristic marine fauna. Only a partial 
resemblance can be instituted with the northern group of coal- 
fields—including Scotland—or with those of the south and south- 
west of England. 
Special points arismg from the examination of the Midland 
sequence and of general significance are: (1) The existence of a floral 
break between the Upper and Lower Carboniferous rocks in North 
