W.M.Sutchings — Clays, Slates, and Coutscf-Metamorplmm. 43 



recorded in published descriptions of the rocks from some of the 

 localities. Thus, among others, notably the andalusite-hornfels of 

 Audlau and neighbourhood is rich in felspar. Also the hornfels of 

 Spitzenberg, in the Harz, the " Knotenglimmerschiefer " of Sau- 

 schwart, in the Erzgebirge, and several others, contain large 

 amounts of newly- formed felspar. My own impression is that in 

 most of the contact-mosaics of normal slates and slaty grits an 

 examination of very thin sections, with adequate optical appliances, 

 ■will show felspar to be present ; that its occurrence at contacts 

 is, in fact, rather the rule than the exception.^ 



The other point to which I wished to recur is that of the nature 

 of the " spots " in the rock at Shap, and in contact-rocks generally. 



The composition and origin of some of the "spots" and "knots" 

 in contact-slates appear to me to be among the least understood 

 of all the deeply interesting things concerned in the processes of 

 contact-metamorphism. Their elucidation is surrounded with difS- 

 culties, but if they could be properly followed out and explained 

 Mve should, I think, be a long way towards knowing what takes 

 place when a rock is metamorphosed by an intruded granite, and 

 what are some of the stages involved in bringing about the changes 

 of which we are obliged to speak rather vaguely as re-crystallization, 

 regeneration, molecular rearrangement, etc., etc. 



That very little can be regarded as settled concerning the spots is 

 evident from the very different descriptions and explanations, given 

 by various observers, as to their nature and contents. 



In some cases the spots (knoten) are stated to be imperfect crystals 

 ^of definite minerals, or to consist of secondary products due to the 

 decomposition of such minerals. Cordierite and andalusite, and 

 their decomposition-products, are thus described as forming the 

 spots, and we may add also white mica, and sometimes felspar, as 

 playing the same part. 



But it does not seem either necessary or desirable to speak of such 

 definite mineral-occurrences under the general and vague name of 

 " spots " ; and, indeed, in those outer zones of contact-slates where 

 the spots are most abundant and characteristic, these definite mineral 

 grains or "imperfect crystals" are more usually absent, or only 

 slightly represented. 



Such large grains, or imperfect crystals, of definite minerals do 

 occur in some parts of some contact-zones together with the inore 

 indefinite "spots" we are now considering, and as they are frequently 



^ In some contact-rocks the felspar is so developed that it is only a question of a 

 glance through the microscope, even with low powers, to make sure of its presence in 

 quantity and that it is a new formation. Thus, in some of the contact-slates and 

 grits (greywackes) of the Elbe Valley and the Lausitz district of Saxony there are 

 large grains of well-cleaved orthoclase and well-twinned plagioclase, whose enclosures 

 of biotite, quartz, etc., place their nature as contact-minerals beyond question. It is 

 in the limpid and often tine-grained ' ' mosaic ' ' of such rocks that the presence of 

 felspar appears to have been often overlooked, and in which, nevertheless, I consider 

 its presence can nearly always be demonstrated. 



It may be of use to mention that a good series of specimens of the most interesting 

 contact-rocks alluded to above, from Saxony, can be obtained from the mineral depot 

 at the Mining College at Freiberg. 



