420 Dr. C. CaUaicay — Mow to Work the Archcean Rocks. 



Suppose, now, that we have a model of tlie earth's crust composed 

 of a layer of quartz or glass below and a layer of calotte above, with 

 a layer of some imperfectly cleavable mineral like felspar between ; 

 and suppose further that this is subjected to strains so as to produce 

 transverse fractures. It is perfectly evident that the character of 

 the miniature fissures thus formed will vary with the depth, being 

 almost mathematically regular in the calcite, and very irregular in 

 the quartz or glass. 



But we know, by the outlines of the coarsely crystalline, i.e. the 

 originally deep-seated, exotics, that the fractures produced at great 

 depths in the earth's crust are highly irregular ; and I believe that 

 we find the best, if not the only, explanation of this well-established 

 fact in the supposition that the actual structure of the crust is fairly 

 represented by our imaginary model, the crystalline cleavage in the 

 latter corresponding to the jointing of the former. 



In other words, we know that fissures formed in the earth's crust 

 are regular near the surface and irregular at considerable depths. 

 The superficial regularity is clearly attributable to the joint-struc- 

 ture ; while the greater irregularity in the deeper portions of the 

 crust is a plain indication that the joint-structure fades away down- 

 wards. Certainly, in view of the marked influence of the jointing 

 upon the forms of fissures near the surface, it is difficult, if not 

 impossible, to conceive how the extremely irregular fractures pro- 

 duced nearer the source of eruptive rocks could have arisen in well- 

 jointed formations. 



It is important, however, in seeking an explanation of the ex- 

 tremely ragged outlines of many granitic masses, to recognize, 

 besides the absence of jointing, the toughening effect of high 

 temperatures, which produce incijDient plasticity and cause the un- 

 jointed rock to tear rather than break. 



VI. — How TO Work in the Arch^an Eooks. 



By C. Callaway, M.A., D.Sc. (Lond.), F.G.S. 



[Concluded from p. 353.) 



THE mineral composition of rocks, often an important test in com- 

 paratively unfossiliferous formations, such as the Trias, becomes 

 of supreme value amongst the Arch^an groups. It is very frequently 

 the only kind of evidence available, and, used with due caution, may 

 lead to decisive results. In Anglesey, as illustrated in Fig. 1, and 

 already described, it is nearly as conclusive as the testimony of 

 organic remains ; for the great similarity of the dark schist in two 

 areas so near together renders their correlation in the very highest 

 degree probable. The gneissic rocks of the Malvern Hills have 

 been classed as Laurentian on the ground of the lithological resem- 

 blance between the two groups, though 3000 miles apart ; and the 

 determination has been generally accepted. But in such a case as 

 this, high probability is all we can expect. Other gneissic and 

 schistose systems have been described in America, though none of 

 them so closely resemble the Malvern series as the Laurentian. 



