128 W. M. H^itchings — An Interesting Contact-Rock. 



colour, with no remains of the qnartz-grains of the rock except the 

 little residual blebs and grains, and streaks. 



Where such largely or fully-developed nodules of adinole are 

 seen we have also a formation of the same material in the main 

 mass of the rock, among the clastic quartz-grains ; but the action in 

 these nodules has always been, as it were, intensified. We do not 

 see in the main mass any such areas of complete dissolving of the 

 quartz, and complete development of adinole, comparable to those of 

 some of the large nodules. 



It may be mentioned that in addition to this exposure at Falcon 

 Glints, there is another occurrence of nodules known in the contacts 

 of the Whin Sill. It is seen exposed at Eowutree Beck, and is here 

 again in a bed of altered shale, 24 feet below the junction. They 

 ai'e of larger size than those described, having longest diameters of 

 as much as 2 inches, and are more flattened in shape. One such 

 large nodule, studied in thin sections, shows that the outer portion 

 is again composed mainly of adinole, while the inner part is 

 a mosaic of quartz, perfectly " regenerated," the grains inter- 

 locking and containing abundance of the enclosures of little 

 rounded fused-looking globules, so characteristic in typical contact- 

 mosaics. Dispersed through the nodule is a large amount of 

 antbrophyllite, and there is also a good deal of chlorite (delessite) 

 which has resulted from its decomposition. 



These large nodules differ in some respects of detail from the 

 former examples ; but there can be no doubt that they owe their 

 formation to the same processes and respresent the drawing together 

 and concentric deposition of a substance formed in the shale during 

 contact-metamorphism, the action of the substance having been also 

 intensified and more highly developed owing to such concentration. 



In my former paper on contact-action, I spoke of a material which 

 can be seen in among the minerals of some altered slates at granite- 

 contacts. In its most typical form of occurrence it shows a 

 characteristic granular structure in ordinary light, and with crossed 

 nicols it shows either a minutely speckly polarization, or stages of 

 the development of this into white mica and other minerals. I 

 pointed out that this material is perfectly neic, bearing no relation- 

 ship to anything which existed in the unaltered slates or shales, and 

 that its appearance is so very characteristic that once remarked and 

 studied it can never be overlooked. My suggestion was that this 

 material also represents in these rocks the residues of the dense 

 solutions, or aqueous fusions, of some of the constituents, such 

 solutions having acted as solvents and transformers of other com- 

 ponents not originally dissolved. 



This substance, with all its appearances and characteristics as 

 described, is more or less present in many of the altered shales near 

 the Whin SiU. It is seen in varying amount in the rock I have 

 been describing, though here its development is not very pro- 

 nounced, the isotropic matter being in much greater abundance. It 

 always bears the same relationship to the other minerals as it does 

 at granite-contacts. 



