tuffs, suggesting that the volcanic contribution represents on the average a slightly more basic magma than 8871 and 8873. C. The Vitric and Crystal-Vitric Tuffs of the North Range Beds Type I. Heulandite-bearing rocks Coarse heulanditized vitric to crystal-vitric tuffs occur as subordinate members of the laumontite rocks of the 450 feet thick NR2 beds, and fine-grained vitric tuffs altered to heulandite and laumontite occur as numerous thin beds throughout the North Range series. The coarser heulandite rocks, such as the analysed specimen 8768 (Table VIII), show beautifully preserved vitroclastic structure (Pirsson, 1915), the margins of cuspate ash particles being traced out and emphasized by films of deep blue-green celadonite and chloritic and clay minerals. Glass has been entirely destroyed. Minutely fibro-lamellar and feebly birefringent heulandite is much the most important replacement product. Sometimes it is faintly pink in colour due to he presence of dusty iron oxides precipitated during devitrification. In some cases clear secondary albite replaces part or all of the glass in a given shard, heulandite replacing the rest and in one exceptional heulanditized tuff (8770), calcite is abundant. Staining proves that many devitrified or finely crystalline shards and lapilli, especially the larger ones, are rich in potash feldspar. Whether they occur as isolated crystals or as phenocrysts in scattered lithic fragments, the majority of plagioclase grains are found to be clear and glassy. They represent the phenocrysts of the exploding magma in their unaltered condition. In the analysed rock, 8768, plagioclase ranges in composition from An36 to An51 and often shows oscillatory zoning, but in the otherwise similar rocks, 8767, 8772, compositions vary about a distinctly more sodic mean, ranging from An29 to An38. suggesting that the 450ft. bed contains tuffs erupted at slightly different stages of magmatic evolution although always of acid andesitic aspect. Other plagioclases in the rock are cloudy and albitic, probably due in part to incipient alteration in place and in part to an admixture of earlier formed albite. Free crystals of augite have the optical properties 2V = 50 — 53 ½°, β = 1.696 — 1.700, closely comparable to those from the augite andesite conglomerate pebble 9847. Hornblende is much less common and accessory apatite, zircon and quartz are found, the latter, at least in part, being of older, metamorphic origin. The thin-bedded fine vitric tuffs with heulandite such as the analysed specimen 8776 (Table VIII), and a similar one containing fragmentary plant remains, 8766, call for little special mention. As is to be expected in very fine-grained, well-sorted sediments, stony fragments and heavy minerals are less conspicuous than in the coarser tuffs. On the other hand tiny wisps of decomposing biotite are noticeable. The heulandite tuffs are of special interest as they are the least altered of the vitric tuffs available for study. Apart from accessory hornblende, the associated phenocrysts are comparable to those of the andesitic lithic tuffs and phenocrystic quartz appears to be absent. Subaqueous transport and sorting of tuffs can bring about a compositional differentiation to some extent, irrespective of changes in the composition of the parent magma, but nevertheless, the higher SiO2 and lower TiO2 and A12O3 of