F. H.Lahee — Metamorphism and Geological Structure. 361 



In many respects the succeeding stages of metamorphism in 

 the psammites are like those in the pelites. Tims, the tokens 

 of advancing metamorphism, as observed in Stages B to D, are 

 (1) an increase in the quantity of sericite (here formed largely 

 from clastic feldspar grains) ; (2) the acquisition of dimensional 

 parallelism by this sericite ; (3) the consequent origin of a 

 secondary rock cleavage ; (4) the transformation of f ragmen tal 

 quartz grains, by a process of granulation (see figs. 6 to 9) suc- 

 ceeding a state of mottled extinction (fig. 5), into lenticular 

 aggregates (H^. 10) of small, somewhat elongate, parallel gran- 

 ules, and (5) the resulting improvement of the rock cleavage ; 

 (6) the further lengthening of the quartz grains by recrystalli- 

 zation (figs. 11 and 12); (7) the growth of such new minerals 

 as ilmenite, biotite, garnet, hornblende (fig. 19), and zoisite ; 

 and (8) the acquisition of dimensional parallelism by ilmenite 

 and later by biotite, first as aggregates (fig. 17) and then as 

 individual plates. 



These changes take place in the same order and under the 

 same circumstances as those which were described for the 

 pelites. Yet, on the whole, any given degree of perfection of 

 the secondary cleavage is attained somewhat earlier in the 

 pelites than in the psammites. There is no necessity for 

 explaining the characters of the several stages in detail. 



jPsephites. — Although stratigraphically the conglomerates 

 and their derivatives have as wide a range as the psammites, 

 their total thickness is less. Their largest expression is in the 

 Dighton or Purgatory conglomerate, the uppermost member of 

 the Carboniferous formation in this region. 



Consisting of both matrix and pebbles, the psephites may be 

 studied from two standpoints ; but since the matrix, in all 

 stages of metamorphism, resembles the psammites in the same 

 stages, we may confine ourselves chiefly to describing the 

 nature of the pebbles. A considerable majority of these are 

 of quartzite. Granite and vein quartz are also not uncommon, 

 and locally, at certain horizons, there are abundant, rather 

 angular fragments of carbonaceous shale, probably intraforma- 

 tional. The last are relatively soft, and therefore, as might 

 be expected, they are somewhat compressed even in the least 

 crushed specimens (Stage A). As usual, this early phase of 

 metamorphism is marked by scarcity of sericite and by absence 

 of a secondary rock cleavage. 



Pebbles of Stage B differ from those of Stage A in being 

 thinly coated with sericite, which gives a silky luster to their 

 surfaces. At the same time there is more sericite in the 

 matrix. 



A further increase in the development of this mica is exhib- 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 196.— April, 1912. 

 24 



