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Show FEAGMENTAL VOLCANIC EOCKSâ€"CONGLOMEEATES. 75 limited in weight to a few ounces and show a sorting or selection of sizes. But in most cases the sizes have a much wider range. Geologists have been in the habit of distinguishing two classes of the coarser fragmental beds. First, volcanic conglomerates; second, volcanic agglomerates or breccias. The conglomerates contain fragments more or less rounded by attrition, which is held to be an indication that they have been gathered together and arranged by the action of the water. The breccias contain fragments which are angular and are presumed to have been showered down around the vents from which they are supposed to have been projected. Beds corresponding to both classes are abundant in the High Plateaus and of very great thickness and area. But I am disposed to accept the conclusion that they have all had a similar origin, and that the projection of fragments from active vents and their descent in a mitraille has had very little to do with their accumulation. As a rule, nearly all of the fragments show comparatively little abrasion Some, indeed, are considerably wrorn; most of them are very little rounded at the angles of fracture, and a great proportion are in a condition in which it is difficult to say whether they have been abraded slightly or not at all; for when detached from the matrix the surfaces are corroded by some action which may have been weathering prior to their final burial or the solvent action of percolating water after their burial and prior to the consolidation of the stratum. None of the fragments exhibit the sharp edges formed by fresh surfaces of fracture. Thus, while well rounded fragments (like those of glacial drift or stream gravel) are uncommon, it is not certain that any notable proportion have been absolutely free from attrition. The average amount of attrition is generally smallâ€"far less than in conglomerates usually occurring in a regular system of fossiliferous or stratified rocks. No sharp distinction can be drawn between those beds of which the included fragments exhibit a considerable amount of abrasion and those in. which no abrasion can be clearly proven. There is every degree of this action and every shade of transition Thus it becomes impracticable to draw any line here between conglomerates and breccias. It has seemed to me that the small amount of abrasion in the conglomerate fragments is susceptible of a partial explanation. The well- |
