Papers read before Section C [Geology). 513 



met with ■which differs in some respects from the varieties common in 

 the district. In its most typical development it shows a large number 

 of dark green rounded spots set in a fine felt of paler green colour, 

 full of glancing needles. Most of the specimens show a more or 

 less clearly defined banding or schistosity. The aflSnities of the rock 

 are somewhat obscure in hand-specimens. But under the microscope the 

 dark spots are seen to represent olivine, which is still partly unaltered, 

 but partly serpentinized, as well as granulitized and drawn out. The 

 rest of the rock is largely made of actinolite in small crystal flakes ; 

 there are also present a green spinel, abundant magnetite, and some- 

 times other ingredients. In most varieties there is no trace of felspar. 

 This' rock passes into varieties in which the spots are much less obvious, 

 and finally grades into a rock that might be described as an. 

 actinolite-schist . 



The exposures 6f this rock, so far as they have been mapped, lie 

 mostly on the outer fringe of the basic intrusion, and therefore close 

 to the line of junction with the metamorphic rocks into which the igneous 

 rocks have forced their way. From its field relations and its general 

 characters it is clear that it represents a type of the basic rock which 

 has undergone aotinolitization on an extensive scale. The original rock 

 from which it is derived must in most cases have been an olivine- 

 enstatite rock, more or less completely serpentinized, or else a very 

 basic troctolite. Although actinolitization is widespread in this, as in 

 other metamorphic areas, a rock of this particular type has not been 

 noted in this locality before. It has nowhere been found in large mass, 

 and seems essentially a marginal phase. The most prominent exposure 

 yet met with is, or was, a clump in a field adjoining the Udny Road, 

 inamediately south of ' Skelly-hill Wood '. This exposure has recently 

 been entirely blasted away, and it seemed desirable to make a definite 

 record of its occurrence. 



VI. — The Aech^an Eocks of Lewis.' By B. N. Peach, LL.D., F.E.S., 

 and J. HOENE, LL.D., F.E.S. 



DUEING- 1911 the authors visited Lewis with the view of comparing 

 its Archaean rocks, previously described by Macculloch, Murchison, 

 Heddle, and James Geiliie, with the types of Lewisian G-neiss mapped 

 by the Geological Survey along the western seaboard of Sutherland and 

 Eoss. The areas examined comprised sections taken at intervals along 

 the east coast from Tolsta Head, north of Stornoway, to near Loch 

 BhroUum, opposite the Shiant Isles — -a distance of about thirty miles ; 

 and along the west side from the Butt of Lewis to Carloway — a distance 

 of twenty-five miles. Traverses were made across the island (1) from 

 Barvas on the west to Stornoway on the east, thence over the Eye 

 peninsula to Tiumpan Head ; and (2) from Carloway by Callernish 

 to Keose on Loch Erisort, and Stornoway. 



A large series of specimens was collected and submitted to Dr. Flett 

 for examination, who has furnished a valuable detailed report showina- 

 wherein they resemble and wherein they differ from types described by 

 Dr. Teall in the Geological Survey Memoir on TJie Geological 

 Structure of the North-West Highlands of Scotland' (1907). Dr. Flett 

 has arranged the specimens in the following groups : (1) muscovite- 

 biotite-gneiss, (2) biotite-gneiss, (3) biotite-hornblende-gneiss, (4) horn- 

 blende-gneiss, (5) pyroxene-gneiss, (6) hornblende-schist, (7) pyroxenite, 

 (8) pegmatite-gneiss, (9) granite-gneiss, (10) mylonite. 



In the various sections examined throughout the island, rocks belonging 

 to groups (2), (3), and (4) are the main components of the Archaean 



^ Abstract of paper read before the British Association, Section C (Geology), 

 Dundee, September, 1912. 



DECADE V. — VOL. IX. — NO. XI. 38 



