TF. M. Hutchings — Clays, Slates, and Contact- Metamorphism. 71 



It is to be noted that during the stage of metamorphism which 

 develops the mosaic of quartz, or quartz and felspar, the biotite 

 formed at an earlier, less intense, stage undergoes further alteration. 

 It may be seen (as at Shap) that the earlier biotite is more ragged 

 and indefinite in form, is less dichroic and less bi-refractive than that 

 which exists in the mosaics nearer contact. Whether it is redissolved 

 and recrystallized in part it does not seem possible to decide ; but 

 the biotite which is enclosed in the quartz and felspar is of much 

 interest, and may give us some indications as to the degree of 

 temperature which prevailed during the processes. In addition to 

 small flakes and more or less definite crystals, we see numerous 

 oval grains and little balls of biotite, whose forms seem to indicate 

 that they were quite plastic at the time of inclosion, and we may 

 well assume that in these zones there was a condition of aqueous 

 fusion of sufficient intensity to melt, or soften, portions of the biotite. 



The interesting question arises as to the mode of origin of this 

 new felspar : What is it made from ? 



It may, I consider, be stated as a fact that neio felspar does not 

 exist in any normal slates, no matter how highly developed they 

 are. I have given much time and patience in studying this special 

 point, and have convinced myself that no regeneration of felspar 

 takes place. In the Cornish slates no trace of felspar of any kind 

 is seen, though these are so very highly developed. Renard records 

 the same absence of it in the very similar Ardennes rocks. 



In Welsh slates a few grains are sometimes seen of clastic felspar 

 more or less altered, and this is seen in other slates also, but such 

 gi'ains are rare and unevenly disseminated. At Shap, as before 

 stated, no trace of it is seen in the less altered slates, though it is 

 so abundant in the well developed mosaics. We have, therefore, 

 in most cases not to deal with a recrystallization of an already 

 existing mineral, but with the formation of this mineral from other 

 materials. The average and normal slate is composed of the 

 materials which make up the clays with which I dealt in the first 

 part of this paper. The simplest explanation, and indeed perhaps 

 the only one open to us on the evidence in our possession, is that the 

 materials of the mica and the quartz of the slates combine, under 

 the conditions we are supposing, to form felspar. In its decay 

 felspar yields mica and free silica, and it is perfectly simple and 

 easy to comprehend how, under the conditions of temperature 

 pressure, etc., we are supposing, with the intervention of such a 

 dense mineral solution as I have shown cause for believing to exist, 

 the reverse process may take place, and we may have a partial 



tained. It may be well seen in igneous rocks. Thus, General McMahon shows it 

 to occur in the marginal portions of Dartmoor granite (Q.J.G.S. vol. 49, p. 388), 

 and has previously shown its occurrence in granite of the Himalayas. It is also, as 

 is well known, seen perfectly developed in the groundmass of some quartz-porphyries, 

 not to be distinguished, as to its structure and general appearance, or in anything 

 except its inclusions, from some of the finer-grained mosaics of altered slates. In 

 considering this structure in the contact-metamorphism of slates it is a point of some 

 importauce that exactly similar structures can be seen to result during the con- 

 solidation of purely igneous magmas. 



