7 Tuffs and agglomerates exhibit wider variations in composition and appearance than do the lavas. Some fine-grained, siliceous tuff closely resembles chert and other massive, coarse crystal varieties are very similar in general appearance to the lavas. Fragments in the agglomerates vary in size from blocks 5 or 6 feet square to pieces of microscopic dimensions. The agglomerates are stratified in a few places and the fragments then are fairly well rounded, but more often there is a chaotic arrangement of large and small angular blocks in a tuffaceous matrix. Fragments of sedimentary rocks, andesite, andesite porphyry, and a fine-grained feldspar porphyry, are contained in the agglomerates. The tuffaceous matrix of the agglomerates contains feldspar fragments and glass in addition to small rock fragments. The sedimentary rocks include thinly bedded argillite, chert, slate, quartzite, sandstone, greywacke, conglomerate, and occasional thin beds of limestone. Many of these rocks are quite similar to members of the Dease series, but are distinguished from the latter by their close associa- tion with voleanic rocks. Well-stratified conglomerates containing pebbles and cobbles of volcanic rocks occur at two localities in Hotailuh moun- tains. In the canyon of Gnat creek, about 1 mile above its junction with Tanzilla river, coarse-grained sediments are interbedded with flows. East of McBride river, in the southeast corner of the area, a conglomerate con- taining pebbles of limestone, chert, jasper, and volcanic rocks is inter- calated with massive agglomerates. Greywacke east of the mouth of Eagle river contains grains of volcanic rock and of augite porphyrite. However, sediments in the northern part of the area occur only in small amount and appear to be restricted to the top and bottom of the group. Both the voleanic and sedimentary rocks have been intruded by granite and by a basic rock now largely altered to serpentine, and are slightly to highly metamorphosed adjacent to the larger masses of granite. Selective replacement of the volcanic rocks by granite has, in some places, developed a pronounced gneissic banding. In other places, the intruded rocks, at the contacts, are schistose, and contain small amounts of chalco- pyrite and pyrite. A zone of sheared and mineralized volcanic rocks extends in a north- west direction along the northern front of Hotailuh mountains for approxi- mately 8 miles. In places, the rocks in this zone are slightly mineralized andesites and andesite porphyries and in other localities, where the alter- ation has been more complete, the rocks are tale schists. Most of the schists contain appreciable amounts of pyrite and, consequently, weather in various shades of red and brown. STRUCTURE In a broad way the structure of the northern part of the area is simple. Northeast of the Cassiar batholith the Dease and McLeod series are folded into a broad syncline which is succeeded to the northeast by a broad anticline. The distance between the axes of the two folds is about 14 miles. The limbs of the main folds have moderate dips, but locally the strata have been crumpled and bent into minor folds. The folds trend northwest and their axes are practically horizontal except at the western