88 



were found to be the richest in the heavy metals, and to contain the greatest 

 variety. Lithia-micas (lepidolite, zinnwaldite and the dark- coloured 

 rabenglimmer of Breithaupt) always contained tin, and with the exception of 

 lepidolite, arsenic, copper and bismuth ; many of them contained also uranium. 

 Potash micas were found to be poor in the heavy metals, copper being the 

 only one which was commonly present. Arsenic, lead, zinc, copper, antimony, 

 cobalt, nickel and silver were detected in the black micas of certain granites 

 and gneisses. The black mica of the so-called propylite of Schemnitz was 

 proved to be especially rich in arsenic, lead and zinc ; and it contained also 

 copper, cobalt and antimony. The rubellan of certain basalts contained 

 antimony, lead, copper and cobalt in considerable quantities, and traces also of 

 bismuth. BECKER (1) has proved the existence of silver and gold in the un- 

 altered augite of the igneous rocks in which the Comstock lode is situated. 



These observations show that the so-called rare metals are very widely 

 distributed, and that they can only be said to be rare in the sense that they 

 are not present, as a rule, in sufficient quantity to be detected by the ordinary 

 methods of analysis. The importance of this fact with reference to the origin 

 of mineral veins is obvious. (2) 



As a rule the chemical elements are not found in igneous rocks in the 

 uncombined state. They usually form complex chemical compounds of which 

 the silicates are by far the most important. Native iron has, however, been 

 detected in the basalts of the north of Ireland by Dr. ANDREWS (3) ; and it occurs 

 in large and small masses, alloyed with nickel and cobalt, in similar rocks 

 from the west of Greenland/ 4 ' 



In discussing the chemical characters of igneous rocks in greater detail it 

 will be convenient to use the old chemical nomenclature, and speak of them 

 as composed of the oxides; silica, alumina, lime, magnesia, &c. Chemical 

 analyses are always expressed in accordance with this nomenclature. Consider 

 now the distribution of the different constituents in the different varieties of 

 unaltered igneous rocks. 



Silica varies from about 40 p.c. in certain olivine rocks (dunite) to 

 about 80 p.c. in certain liparites or rhyolites. If the analyses of a 

 large number of igneous rocks be compared it will be seen that they may 

 be arranged in a continuous series, so that the silica percentage varies 

 gradually and continuously from one extreme to the other. Variation in 

 silica-percentage is accompanied by variation in the other chemical and 

 physical properties. 



Alumina varies within much narrower limits, viz., from almost nothing 

 to about 20 p.c., if we include the olivine rocks ; and from about 10 to 20 p.c. 

 if we exclude these rocks. 



Ferric oxide, ferrous oxide, magnesia and lime exhibit wide divergencies 

 in their relative proportions divergencies which must be explained by local 



(1") Geology of the Comstock Lode, U.S. Geological Survey. Monographs, Vol. III. 



(2) The above account of elements already recognised as occurring in igneous rocks does not 

 profess to be exhaustive. 



(3) British Association Report, 185, (pt. 2), p. 34. 



(4) STEENSTETTP. On the existence of Nickel-iron with WIDMANSTATTEN'S figures, in 

 the basalt of North Greenland. M.M., Vol. VI., page I. 



