PEESSUEE IN STEATiriED PALEOZOIC EOCES. 15 



resembles a matted mass of minute fibrolite ; but after examination 

 •with high powers, I incline to consider it only an aggregate of 

 very minute flakes of colourless mica. The old lines of the 

 strain-slip cleavage are occupied by a filmy brownish mica asso- 

 ciated with opacite ; the flakes of this lie roughly along the planes 

 and are wavy, sometimes almost intertwined, like a section of an 

 imperfectly twisted cord. 



It is evident that these rocks have been not only stratified but 

 also much disturbed, subsequent to stratification, though prior to a 

 large part of their alteration. We have the usual waving, arching, 

 and puckering of the diverse mineral layers ; we have even the trace 

 of a strain-slip cleavage, as above described ; but in places where 

 the bands of mica are forced into sharp angular folds there is no 

 sign of the nipping, creasing, or tearing which are familiar to 

 everyone who has examined foliated rocks which have been subse- 

 quently subjected to folding by pressure. The mica flakes are well 

 developed ; true, they are sometimes interrupted thus /\, some- 

 times bent thus O at a sharp fold, but in each case the flakes are 

 well developed, as though they either had been formed, or had 

 grown larger, and perfected their outline on the spot. The latter, 

 I have no doubt, is the true explanation. The rock, after under- 

 going severe pressure, resembled in its incipient metamorphism those 

 first described. The results of the intrusion of a molten mass set 

 up further chemical action, caused both destruction and construction. 

 The residual detrital stuff yet remaining in the rock — the " dirt,'' 

 we may call it — was obliged to make itself useful ; silica went to 

 enlarge quartz granules, some of the ferruginous compounds produced 

 magnetite, haematite, limonite, or other iron salts ; and magnesian 

 or alkaline earths went to aggrandize the micas already existing ory 

 in some cases, to form new flakes ; carbonaceous matter perhaps 

 sometimes was oxidized, sometimes became graphite. Thus the result 

 of the action of a mass of heated rock on a rock previously banded 

 has been to produce a very fair imitation of an ordinary fine-banded 

 mica-schist. Differences between these will be presently noticed. 



The crystalline grains of andalusite in No. 3 are of moderate size, 

 from about '3" by •!" downwards. Under the microscope they are 

 seen to be associated with quartz, brown mica, ferrite, and opacite. 

 The last two are abundant enough to give a rather dusty look to 

 the mineral; they traverse the crystals in bent or wavy bands, 

 obviously indicating the direction of the (contorted) original stratifi- 

 cation; and the somewhat " crippled" outline of the crystals intimates 

 that their full development has been interfered with by the banded 

 structure of the rock. 



In order to eliminate the effects of the puckering, I examined 

 a specimen from another locality on the same road, about 7 kil, 

 from Morlaix. This is a fine-grained, somewhat micaceous rock, 

 looking rather like an altered greywacke, with a much less clearly 

 marked foliation, which presumably corresponds with an original 

 bedding and shows no puckering. The general character of the 

 rock is very similar, except that it is rather distinctly foliated. The 



