Mica and hornblende, although the principal, are not the only, accessory minerals in gneiss; but the following species are also of common occurrence: garnet, cyanite, tourmaline, fibrolite, epidote, and chlorite. Gneisses, as the table indicates, exhibit a wide range in the proportion of silica which they contain, varying from 60 to 85 per cent.; and there is a concomitant variation in specific gravity, from about 2.5 in the most acidic to 2.8 in the most basic varieties. That gneiss is a true, stratified rock is very clearly shown in specimen 41; but, unfortunately, the stratification is not always so evident as in this case. The mica-scales, it will be observed, lie parallel with the stratification, and assist very materially to make it visible; and gneisses containing little or no mica, as well as some that are rich in mica, frequently appear almost or quite unstratified. These obscurely stratified varieties are commonly known as granitoid gneiss, having the texture and general aspect of granite. The sedimentary origin of gneiss is also clearly proved by its interstratification with undoubted sedimentary rocks, such as limestone, iron-ores, graphite, quartzite, etc. Syenite.—This is a much abused term, but, as now employed by the best lithologists, it is the name of a rock having a single essential constituent, viz., orthoclase. Syenite in its simplest variety contains nothing but orthoclase; but in addition we usually have either hornblende, forming hornblendic syenite, or mica, forming micaceous syenite. Syenite, it will be observed, is equivalent to gneiss with the quartz removed; but, while gneiss is the most abundant of all rocks, syenite is a comparatively rare rock; and this is simply another way of saying that nearly all orthoclase is associated with quartz. By admixture of quartz we get a perfectly gradual passage from syenite to gneiss. The orthoclase in syenite is more frequently replaced by plagioclase than it is in gneiss. In syenite, too, hornblende is much more abundant than mica; although just the opposite is true in gneiss. And, again, in gneiss the mica is principally muscovite; but in syenite it is almost exclusively biotite. Augite is a common accessory in the more basic syenite; but garnet, tourmaline, and the other accessory minerals, occurring so frequently in gneiss, are almost unknown in syenite. The specific gravity of syenite varies from 2.7 to 2.9.