EMPHASIS OF STRUCTURES AND TEXTURES. 525 



surfaces. Textures which the eye can not discover become clearly apparent. 

 Any variation in the hardness or solubility of the rock is shown by a ridgy 

 structure. As a consequence, every variation in original and secondary 

 structures and textures may be shown on a weathered surface. So nice is 

 weathering in its selective power in disintegration and solution, that when 

 the different unaltered rocks can not be discriminated the weathered forms 

 are easily separable. 



Some cases will be mentioned illustrating the above general statements. 

 Frequently the rocks which when unweathered appear to be massive, in the 

 belt of weathering develop a strongly marked cleavage or fissility — cleavage 

 in the lower part of the belt of weathering and fissility in the upper part. 

 This effect is so marked that often it is difficult to believe that the weathered 

 and unweathered portions of the rock belong to the same formation. The 

 unaltered rock may be so massive that it has only a rift which can scarcely 

 be called cleavage. The weathered portion may be split up into leaf-like 

 layers, scores of them within the breadth of an inch, so that the rock at once 

 would be called a schist, while one would not think of applying the term 

 schist to the massive phases. 



In connection with the emphasis of schistosity, micaceous minerals are 

 likely to become very conspicuous. In the weathered portion of some 

 rocks it seems as if the mica is one of the most abundant of the minerals, 

 while in the unweathered portion it is scarcely noticeable. Excellent 

 illustrations of this are found in A-arious places on the Piedmont Plateau. 

 Perhaps one of the best is at Washington, D. C. Here the unweathered 

 rock at various places is a very massive appearing, slightly cleavable granite 

 or a somewhat dense schist, whereas the partly weathered portions are 

 fissile mica-schists. 



Where mechanical forces have not displaced the particles with reference 

 to one another, a rock may be completely weathered in the sense that none 

 of the original minerals remain, and yet perfectly retain the original struc- 

 tures and textures. This is beautifully illustrated by the decomposed 

 diabase dikes in the Penokee district, which still perfectly retain the textures 

 of diabases, and yet in composition some of them approach very closely to 

 kaolin. Every original mineral has been completely altered. Other 

 similar instances described by Merrill" are the decomposed gneisses and 



o -Merrill, George P., Eocks, rock-weathering and soils, Macmillan Co., New York, 1897, p. 264. 



