Vol. 52.] BOULDERS FROM THE KOLGUEV BEDS. 59 



Under the microscope it is found to consist of an almost micro- 

 crystalline groundmass, in which are scattered larger crystalline 

 grains. This groundmass is composed mainly of quartz and 

 felspar, exhibiting occasionally an approach to a pegmatitic 

 structure, but generally forming a rather irregular mosaic, as if 

 the two minerals had simultaneously crystallized. Interspersed 

 with these, with a rather ' clustered ' habit, are numerous 

 needles, apparently of a darkish actinolite, and occasional 

 flakelets of biotite. The larger (porphyritic) crystals are both 

 quartz and felspar ; the former mineral is not abundant, but it 

 occasionally includes a small grain of felspar or flake of biotite, 

 is sometimes corroded by the groundmass, and is often cracked. 

 The latter mineral, however, exhibits a structure which is not 

 a common one. It contains numerous enclosed grains, often 

 rather rectangular in outline, of another felspar, and sometimes 

 of quartz, like a 'lustre-mottling.' These enclosures occur 

 somewhat sporadically, i. e. they occupy only a part of a crystal. 

 This also, in one or two cases, has evidently increased in size 

 during the last stage of consolidation. We have found, for 

 instance, two planes (perhaps a basal and a clinodome) outlined 

 by minute flakes of biotite, and marking a change in structure, 

 though not in optical continuity. The outer zone, about -£q inch 

 in maximum breadth, contains a larger number of quartz- 

 enclosures, and these are ranged as if they had grown from the 

 original surface of the crystal, somewhat as the crystals in a 

 granite occasionally group themselves at a junction with 

 another rock. This particular crystal also shows an approach 

 to a micropegmatitic structure, which, however, sometimes 

 passes beyond the boundary. These larger felspars, as a rule, 

 do not exhibit the oscillatory twinning of plagioclase ; one or 

 two are orthoclase, twinned on the Carlsbad type, a few pro- 

 bably are microcline. One or two small zircons are present. 

 The rock would be called by some a quartz-felsite or quartz- 

 porphyry, by others a microgranulite. The general structure 

 (except for the peculiarities of the felspar) resembles that of 

 the rock figured by Fouque and Levy, ' Mineralogie Micro- 

 graphique,' pi. x. fig. 1). 

 (21) A darkish, purple-spotted grit, which, under the microscope, 

 proves to be composed of fragments, subangular to fairly 

 rounded, of minerals and rocks. Among the former are quartz 

 and felspar, which in one or two cases are associated as if they 

 came from a holocrystalline rock. Among the latter are man} r 

 fragments of volcanic rocks. These are now de vitrified and 

 often rather decomposed, but they evidently were once in a 

 glassy or slaggy condition. Occasionally they are blackened 

 with opacite ; felspar-microliths are present in some. Appa- 

 rently they represent some of the less acid trachytes — probably 

 andesites ; possibly, in one or two cases, even basalts. These 

 exhibit considerable variety, so that they hardly can have come 

 from a single volcano, but nothing worthy of detailed descrip- 



