146 IF. II Twenhofel — Silurian Section at Arisaig. 



The entire section is much fractured and in many places the 

 rocks have been crushed literally to fragments. In the harder 

 rocks the fractures have been filled by quartz and calcite, being 

 in many places so well cemented that when struck the rocks 

 break elsewhere than along the original line of fracture. As 

 many as twelve cemented fractures have been counted on a 

 fragment three inches wide. This cementation is confined to 

 the harder rocks, being practically absent in the shales. 



The general direction of the fracture lines is, on the average 

 of those taken, about twenty degrees east of north and thirty 

 degrees west of north ; but many variations occur. The section 

 has probably been subjected to pressure more than once in its 

 history. 



The greatest fault found occurs at the top of zone 12, where 

 the lower shales have been elevated and abut against higher 

 limestones. In. this case the direction is thirty-live degrees 

 west of north. 



Some of the joint blocks are very symmetrical. In three 

 beds in which they were particularly regular the following 

 measurements were taken : N. 20° E. by N. 80° W., N. 30° 

 W. by K 70° E., and 1ST. 28° W. by K 70° W. 



JPetrological Divisions. — The rocks can be divided petrolog- 

 ically into the upper shales and limestones having their base 

 on the " Red Stratum," the middle limestones and shales, the 

 middle dark shales, the lower green and dark shales, and the 

 basal shales and arenaceous limestones. These penological 

 divisions foreshadow the paleontological ones, although from 

 the latter standpoint it is difficult to fix boundaries, as no sharp 

 break occurs in the section except at the fault at the top of 

 zone 12. There is a total thickness of thirty-four hundred and 

 sixty-five feet. 



Character and Color of the Rocks. — The rocks comprise 

 shales, ranging from fine-grained carbonaceous paper shales to 

 others decidedly coarse and arenaceous, tough argillaceous lime- 

 stones, rarely pure limestones, flint-like flags, and fine-grained 

 sandstones. In the lower shales are a few beds of low-grade 

 iron ore, once worked but now abandoned. 



The color of many of the rocks on a wave-beaten surface is 

 a dirty green, on a weathered surface a rusty yellow or brown. 

 The shales range through gray, red and black. In the upper 

 part they are red followed by gray and green shades. The thick 

 middle shale horizon is of a dark gray color. The upper half 

 of the lower shales are green ; in the lower half dark to black 

 colors predominate. The sandstones and flinty flags have some 

 shade of blue. The limestones range from gray to grayish- 

 green, the latter predominating. 



