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of first importance in field geology. The subject is still, in 

 some of its aspects, obscure and requires further investigation. 

 It may be useful for such a purpose to supply references to 

 the literature of the subject, with some additional notes based 

 on personal observation. A diversity of terms bearing on the 

 subject has passed into common use, some of which are 

 mentioned below. 



One of the earliest observers of dynamic brecciation was 

 J. E. Marr, who noted these phenomena in the Devonian 

 rocks of Devonshire. (1) In the section described numerous 

 small thrust planes occurred in thin-bedded limestones, 

 causing the break-up of these beds into "eyes" of limestone, 

 the separated fragments measuring from a fraction of an 

 inch to several feet in length. These formed lenticular 

 patches interbedded with the normally cleaved argillaceous 

 material — a schist consisting of alternating lenticular patches 

 of limestone and clay slate. Marr did not suggest any name 

 for this particular form of brecciation. 



Litholog-ical Nomenclature. 



AUTOCLASTIC. 



Mr. H. L. Smythe, in an article on "The Geology of Steep 

 Rock Lake, Ontario, "(2) ^^ed the terms "autoclastio schist" to 

 distinguish certain features of rock structure that came under 

 his notice in the locality mentioned. In a foot-note he 

 explains that by the above terms he means "schists formed 

 in place from massive rocks by crushing and squeezing without 

 intervening processes of disintegration or erosion, removal, 

 and deposition." Similar effects have been noted by other 

 observers that are not of a schistose character. Usually it 

 follows as an effect of tangential pressure that causes one 

 rock mass to slide over another by which a zone of brecciation 

 occurs, the clastic fragments being of the same kind as the 

 rock immediately above or below the thrust plane. 



Mr. Van Hise, in his important work on "The Principles 

 of North American Pre-Cambrian Geology," uses further dis- 

 tinguishing terms with respect to this kind of structure, and 

 says, (3) "When rocks are folded by strong erogenic forces, 

 and they are not so heavily loaded as to render them plastic, 

 they are frequently broken into fragments and 'autoclastic* 

 rocks are produced. The autoclastic rocks that readily show 



(1) Marr, "Some Effects of Pressure on the Devonian 

 Sedimentary Rocks of North Devon," Geolog. Mag., 1888, p. 218. 



(2) Am. Jour. Science, 3rd ser., vol. xlii. (1891), p. 331. 



(3) United States Geolog. Sur., 16 Ann. Reports, part i., 

 1896, p. 679. 



