HYPOGEIC WORK. 371 



by simple pressure; and later Daubree, who experimented with clay and 

 scales of mica, obtained a perfect schistose structure. The rolling and ham- 

 mering of metals result in a laminated texture, which fracturing or acids 

 may reveal, when not otherwise visible ; and several fine examples are fig- 

 ured by Daubree in his excellent work on Experimental Geology. 



Mountain-making was going forward, and the work done was therefore on 

 a large scale, producing at one effort slaty structure over areas of hundreds 

 of square miles, with great uniformity of direction and high angle of pitch. 

 Sedgwick recognized the approximate coincidence of the strike of the slates 

 with the strike of the beds, or rather, as Professor Phillips stated it, with 

 the direction of the main axis of elevation. The uniformity of product and 

 evenness of surface are a consequence of the fineness and evenness of grains 

 of the original argillaceous formation, and the regularity of the long-con- 

 tinued pressure ; but partly also of the moderate degree of heat during the 

 action of the pressure. 



Purther : pressure has been proved to have produced a foliated, and even 

 a schistose, structure in the granite-like rock, of igneous origin, called 

 granulyte, and also in augitic and other igneous rocks. 



A slaty formation often contains fossils, and these indicate, to some extent, the 

 degree of compression and distortion which the beds containing them underwent under the 

 pressure. The fossils in Fig. 344 are from a paper on slaty cleavage. This subject has 

 been treated mathematically by Professor Haughton (1846, 1857); and more recently by 

 A. Barker (British Association, 1885). 



Slaty cleavage, or that characterizing roofing slates, passes gradually into the foliation 

 of hydromica schist and mica schist, and thence into that of gneiss and gneissoid granite, 

 suggesting that the latter may be due in these rocks to pressure. This has been confirmed 

 by experiment and observation. But geological observation is required to settle any 

 doubts that arise, rather than the microscope. In general, the foliation of mica schist 

 and gneiss is not a result of pressure, but, on the contrary, of the original bedding of the 

 formation. The evidence of this often appears in the occurrence of large variations in 

 strike and dip in the planes of foliation, instead of the high angle and evenness character- 

 izing slates ; in flexures of the sheets of rock, anticlinal or synclinal ; and in alternations 

 of the sheets with those of limestone or other kinds of rock, such alternations in connec- 

 tion with low dips or flexures being good evidence that the sheets are true beds. Only 

 the finer kinds of metamorphic rocks — argilly te and hydromica schist — often lose their 

 bedding by the substitution of the cleavage structure through pressure. 



4. Joints. — Joints in rocks (see page 111) have various methods of 

 origin. They are in part due to slow-acting pressure on the outskirts of a 

 region of disturbance. The pressure may act with little or no warping of 

 the beds. That this is often the case is indicated by the general parallelism 

 in the joints. But in other cases warping or torsion is strongly marked, as 

 Daubree has shown. Daubree has illustrated the effects of torsion on the 

 courses of joints by subjecting plates of ice to the action. He obtained, as 

 one of his results, with a plate nearly a yard long, the fractures shown on 

 a much reduced scale in Pig. 345. Pig. 346 shows a portion of one of the 

 plates one fourth of the natural size. (It is from a photograph, and lience 



