624 STUDIES FOR STUDENTS 



AUTOCLASTIC ROCKS. 

 ORIGIN OF AUTOCLASTIC ROCKS. 



When rocks are folded by strong orogenic forces, and they 

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

 frequently broken into fragments, and '' auto clastic"'' rocks are 

 produced. The autoclastic rocks which readily show their 

 origin may be called dyiianiic breccias, and those which resemble 

 ordinary conglomerates may be called pseiido-cojiglomerates. 

 Brittle rocks are the most likely to become autoclastic ; hence 

 it is that cherts, quartzites, cherty limestones, graywackes, and 

 rather siliceous slates are some of the kinds which most fre- 

 quently present the phenomena described. The movements of 

 the broken fragments over one another in many cases so thor- 

 oughly round them that they have the appearance of being 

 waterworn, and the matrix between the larger fragments may 

 consist almost wholly of well-rounded fragments of a similar 

 character. For instance, in a semi-indurated quartzite the larger 

 complex fragments may be well rounded by their mutual fric- 

 tion while the matrix may consist of the simple original water- 

 worn grains which are rent apart. In another case the original 

 rock may have consisted of beds of mud interlaminated with 

 thin beds of grit. By consolidation and cementation these beds 

 may have been transformed to alternating shale and graywacke. 

 The shale is plastic under slight load ; under the same load the 

 graywacke is brittle. When such a set of beds is deformed 

 the shale yields largely by flow and the graywacke by fracture. 

 The beds of graywacke are broken into fragments of varying 

 sizes, which are ground over one another, and thus are rounded. 

 At the same time the shale flows and fills the spaces between 

 the fragments. Also slaty cleavage may be developed. As a 

 result, a pseudo-slate-conglomerate is produced, having a slate 

 matrix and pebbles of graywacke, which, so far as its own char- 

 acters are concerned, could not be discriminated by anyone 



' Structural geolog}^ of Steep Rock Lake, Ontario, H. L. Smyth, Am. Jour. Sci., 

 3d sen, Vol. XLII, p. 331. 



