The researches of the Survey have shown the so-called Lewisian gneiss to comprise the following five groups of rock: 1. A group of various more or less banded and foliated rocks which form together the oldest and chief part of the gneiss (Fundamental complex); 2. Highly basic dykes cutting the first group; 3. Dykes and sills of dolerite, epidiorite and hornblende-schist; 4. A few dykes of peculiar composition; 5. Gneissose granite and pegmatite. The first of these groups, forming the main body of the gneiss, has been critically studied on the mainland from Cape Wrath to Skye. But its development in the Outer Hebrides has not yet been worked out, although the name "Lewisian" was actually taken from that chain of islands. So far as at present known, however, the gneiss of the Hebrides repeats the essential characters of that of the mainland. Mr. Teall, as the result of a careful investigation in the field and with the microscope, has ascertained that on the mainland between Skye and Cape Wrath the rocks of the "fundamental complex" are essentially composed of olivine, hypersthene, augite (including diallage), hornblende, biotite, plagioclase, orthoclase, microcline and quartz. He has further observed that these minerals are associated together in the same manner as in peridotites, gabbros, diorites and granites. Treating the rocks in accordance with their composition and partly with their structure, but excluding theoretical considerations, he has arranged them in the following five subdivisions:-- 1. Rocks composed of ferro-magnesian minerals, without felspar or quartz--Pyroxenites, Hornblendites. 2. Rocks in which pyroxenes are the dominating ferro-magnesian constituents, felspar always being present, sometimes quartz: A, Without quartz, Hypersthene-augite-rocks (pyroxene granulites; rocks of the Baltimore-gabbro type) and augite-rocks (gabbros); B, With quartz, Augite-gneiss. 3. Rocks in which hornblende is the prevalent ferro-magnesian constituent: A, Without quartz, or containing it only in small quantity; rocks basic in composition: (_a_) massive or only slightly foliated (Amphibolites, as epidote-amphibolite, zoisite-amphibolite, garnet-amphibolite); (_b_) foliated (Hornblende-schist). B, With quartz; rocks intermediate or acid in composition: (_a_) with compact hornblende and a granular structure (Hornblende-gneiss proper); (_b_) with hornblende occurring in fibrous or other aggregates; (_c_) with compact hornblende and a more or less granulitic structure (Granulitic hornblende-gneiss).