502 FREDERIC H. LAHEE 



Briefly the characters of the four stages may be summarized as 

 follows: 



Stage A: 



Megascopic. — Fracture of rock irregular, or there may be a tendency to 

 break parallel to the bedding. The fracture surfaces are dull. Original 

 structures, fossils, and pebbles are not deformed. 



Microscopic. — Nearly all the constituents are of clastic origin, with clastic 

 outlines. Sericite, the first metamorphic mineral to appear, may be present in 

 small quantity. There is no parallel alignment of elongate or flat minerals, 

 unless parallel to the bedding, and then this arrangement is primary. 



Stage B: 



Megascopic. — There is a fair secondary cleavage. The cleavage surfaces 

 have a faint gloss due to the parallel orientation of microscopic blades of 

 sericite. Original structures, fossils, and pebbles may be somewhat distorted. 



Microscopic. — Clastic grains are bent, strained, or crushed. Quartz grains 

 may be granulated about their edges, or in bands that cross them, or entirely, 

 and in the latter case the aggregates of grains may have been pressed out into 

 lenticular form. Sericite is more abundant than in Stage A and a large pro- 

 portion of it has parallel orientation. It is the principal cleavage-maker. 

 Pebbles in conglomerate are thinly coated with this mica. 



C: 



Megascopic. — The secondary cleavage is very good and the fracture surfaces 

 have a brighter gloss. Fossils are usually unrecognizable. Pebbles may be 

 much flattened or elongated. Original structures have been obscured or 

 destroyed. Metacrysts' begin to be of marked importance. 



Microscopic. — Much of the crushed clastic quartz has been recrystallized, 

 the new grains being somewhat elongate parallel to the schistosity. Feldspar 

 of secondary origin is occasionally seen. Sericite is very abundant and may 

 show signs of growing coarser, to muscovite.^ Its plates and laths lie approxi- 

 mately parallel. In the conglomerates sericite may be observed developed 

 within the pebbles, but most other new minerals occur in the paste only. The 

 mica coatings of the pebbles are thicker. 



Stage D: 



Megascopic. — ^A thin, perfect cleavage characterizes the rock. There may 

 be a false cleavage. Fracture surfaces have a high sheen. Original structures 

 are much deformed. Pebbles are either sheet-like or spindle-shaped, much 

 longer than they are thick. No sign of fossils can be discovered. 



' The name " metacrystals " was proposed by Lane for phenocrysts in metamorphic 

 rocks, these crystals being of later origin than the groundmass (A. C. Lane, " Studies of 

 the Grain of Igneous Intrusives," Bidl. Geol. Soc. Am., XIV [1903], 369). 



2 Grubenmann, op. cit., p. 86. 



