George Barrow — Rocks from the Gavarnie District. 421 



V. — Notes oif some E,ocks feom the Gavarnie District of the 



Pyrenees. 



By George Barrow, F.G.S. 



(PLATE XIX.) 



MR. DIXON has handed me a small collection of rock specimens 

 from Gavarnie (Hautes Pyrenees) and the district to the east. 

 Microscopic sections have been made of these, with the object of 

 ascertaining whether the structures and composition of the rocks throw 

 any light on the vexed question of the age of the metamorphism in 

 the two areas specially dealt with in the preceding communication. 



The igneous rocks from Gavarnie and Heas, both in their general 

 character and mode of occurrence, show the features specially 

 characteristic of very old intrusions. They tend to infinite sub- 

 division, occurring either as thin sills of granite or as veins of more 

 or less foliated pegmatite (giant granite). It is an additional feature 

 of these thin intrusions that in the majority of cases their foliation 

 is of prot;pclastic (pre-consolidation) and not of kataclastic (post- 

 consolidation) age. 



The two specimens, 186 (near Gavarnie) and 189 (Heas Yalley, 

 near the cirque de Troumouse), are typical examples of a sill of 

 granite and a foliated (protoclastic) pegmatite. The first (186) is 

 mainly a typical igneous ' granulite,' in which the grains are of 

 fairly uniform size, and there are no specially large patches of either 

 quartz or felspar, though the junctions of the component grains are 

 essentially the same as those of a normal granite. It is composed 

 of quartz, orthoclase and plagioclase, with a subordinate amount of 

 brown and white mica. The plagioclase is mainly oligoclase, and 

 shows a typical feature of these old rocks, namely, the development 

 within the plagioclase crystals of numerous small flakes of white mica, 

 due probably to their being kept at a high temperature for a long 

 period after they had separated out. The biotite is in irregular 

 shaped patches and often intergrown with muscovite, only a small 

 quantity of the latter being free. 



On the margin of the igneous granulite are films or patches rich in 

 felted brown mica, having the reddish-brown tint of the ' contact 

 mica,' developed in altered sediments by heating. They show clearly 

 the presence of original films of sedimentary material, now intensely 

 altered ; the bulk of the new quartz and felspar associated with the 

 mica-films merges absolutely into the adjacent igneous material, and 

 no trace of a true junction between the two is visible in the 

 microscopic section. This absence of clear junctions is a characteristic 

 feature of lit par lit intrusion in very old rocks. 



In the gneissose pegmatite 189, large crystals were formed, which 

 are now more or less rounded, as well as surrounded by a mixture 

 of fine ' granulite ' material and slightly larger grains. The latter 

 adhere so closely to the large rounded crystals as to suggest they were 

 broken off from the latter during 'pasty-flow,' the structure being, 

 in fact, a typical ' protoclastic foliation.' 



Three specimens of the older altered sediments have been selected 

 for examination. Of these the first, 187 (near Gavarnie), is a hard, 



