ON THE CRYSTALLINE ROCKS OF ANGLESEY. 167 
The undetermined part consists of free carbon and some volatile sul- 
phides. During the grinding process the smell of H.S or some organic 
sulphide was detected, and lead acetate paper was blackened when held 
near the freshly-ground rock. An attempt was made to dry distil the 
rock and condense the vapours evolved, and this was partly successful. 
When the powdered rock was strongly heated fumes were evolved 
having an offensive odour resembling that of petroleum as well as H.S, 
and which were condensed to a pale yellow oil; as, however, only about 
three drops were collected it could not be further examined. 
Further examination of this rock would be of great interest. 
This rock is thin-bedded, compact, and very dark. Corals are rare, 
and indeed fossils generally less abundant than in the other type, except 
in certain seams in which are large numbers of Produclus giganteus. 
Nos. 546,, 5474, have been examined on account of the dolomitic 
appearance that certain beds assume in the neighbourhood of masses of 
coral. 
No. 546A. G'rey Limestone. Carboniferous. 
Penrhyn y Gell. Traeth Bychan. 
T II. 
Residues insoluble in HCl ; ae (i9o 799 
Al,0,+ FeO, . ; s , =) O:62 0°58 
CaQ. : P : ; : . 50°49 50°53 
MeO. ; ‘ : ; : . 069 0-51 
COy-. A ; : $ 3 . 40°42 40°35 
100-15 99-96 
Percentage CaCO,= 90°16 90:23 
This is a massive, lightish grey limestone, of a type that makes up a 
great part of the Carboniferous series. The percentage of insoluble 
residues is higher than might have been expected from the appearance 
of the rock. 
No. 547A. 
From same bed as 5AGA, but close to a coral. 
Ts TT; 
Residues insoluble in- HCl ‘ - O24 018 
Al,O, 5 : 5 , ‘ ~~ O63 0:69 
Fe,0, ’ : : : 4 ee LAB 1:34 
FeO . : : é . ; lasts 5-63 
CaO . 4 t F ‘ 5230525 30°36 
MgO. ; c : E I . 16:22 16°19 
(1 NE ASE SSC Me BME? twee 45°69 
100°12 100°08 
Percentage FeCQ,= 8:99 9:07 
LS CaCO, = 5401 54°19 
3 Mg CO, = 34-06 33:99 
It will be seen that in the neighbourhood of the coral, iron as well 
as magnesium replaces calcium in the carbonate, and that the soluble 
_ residues sink from nearly 8 to less than *5 per cent. 
