TRANSACTIONS OF SECTION C. 397 



Devonian Carboniferous and Permian (younger Palwozoic) ; and th>i Trias, 

 Jurassic, and Cretaceous. The Kainozoic (Tertiary) is indicated by small 

 crosses, diagonal for the Eocene and Oligocene, and upright for the Miocene 

 and Pliocene. In each case t"he earlier division is distinguished by the crosses 

 in adjoining rows (following the direction of the arms) alternating, and the later 

 by crosses opposite each other in such adjoining rows. The Axithropozoic 

 (Quaternary) is shown by rows of small circles or dots, the former being 

 reserved for the Pleistocene and the latter for the Recent. 



Minor divisions may be distinguished (1) by varying the size, thickness, or 

 spacing of the lines or other symbols, (2) by adding new symbols. Where 

 desirable, the recognised symbols of lithological characters may be added to 

 those denoting the formation. Passage Beds between two formations may be 

 shown by a combination of shading. 



For volcanic rocks the symbols employed for sedimentary rocks are very 

 much thickened. When the age of intrusive rocks is known, it may be 

 indicated by the corresponding shading for sedimentary rock, with the white 

 and black interchanged ; or, if preferred, their nature may be shown by white 

 letters on black. 



2. The Acid Rocks of Iceland. By Leonard Hawkes, M.Sc. 



An account was given of the preliminary results of an investigation of the 

 Tertiary acid series. It is known that these rocks are widely developed in East 

 Iceland, but hitherto definite information as to their extent, nature, and mode 

 of occurrence has been lacking. Whilst they have been stated to be partly 

 intrusive and partly extrusive (I., p. 269), it has generally been accepted that 

 they are dominantly intrusive in character (I., p. 232; II., p. 5; III., p. 783), 

 a vieiw which has iprobably been influenced by the general intrusive nature of 

 the British Tertiary acid rocks (IV., p. 364). 



The main exposures of acid rocks in Elast Iceland from Borgarfjord to 

 Berufjord have been studied in the field. Evidence was brought forward to 

 show that these rocks are in the main extrusive in character. In places {e.g. the 

 Borgarfjord district) the acid series is at least 2,000 feet in' thickness. Tuffs 

 and sphaerolitic liparites and obsidians are very common. The author holds that 

 the old view that the acid rocks are dominantly intrusive, being thus marked 

 off from the basic rocks, is incorrect. Tertiary volcunic activity was similar 

 to that which has obtained in Iceland in post-glacial tiines, when acid rocks have 

 been extruded along with the basic, but in a smaller amount. Acid eruptions 

 seem to have taken place almost continuously during the building-up of the 

 Tertiary plateau. The uneroded character of the liparite lava streams shows 

 how rapidly the successive basalts which submerge them were poured out, and 

 this throws some light on the problem of the intrusive or extrusive origin of 

 the Antrim rhyolites. 



Since the close of the Tertiary volcanic period enormous denudation has 

 obtained, and the varying resistance offered to erosive agents by acid and basic 

 rocks has produced some remarkable effects. 



Thoroddsen (I., p. 159) has described some peculiar streams of acid rocks 

 which he regards as post-glacial lava flows, formed by the extrusion of liparite 

 blocks in a half -melted condition from the mountain-sides. The most noteworthy 

 of these occurs in the Lodmundarf jord. The rocks of the district are Tertiary 

 bedded basalts, with the exception of an acid series, contemporaneous with the 

 basalts, revealed in a huge cirque excavation in a side valley. The valley is 

 full of a chaotic assemblage largely composed of sphferolitic liparite reaching 

 down from the cirque (Skumhottur) on- to the bottom of the main (Lodmun- 

 darfjord) valley. The author holds that these blocks do not represent a lava- 

 stream but a moraine. All the rocks of the stream occur in situ in the 

 Skumhottur mountain. The theory of morainic origin has been previously 

 rejected partly on account of the reported exclusive liparite composition where 

 a mixture of acid and basic rocks would be expected. It was found, however, 

 that the stream is not exclusively composed of acid types, though dominantly 

 so. The large proportion of liparite present results from it« lesser resistance 

 to ice-erosion compared with basalt, whereby the huge cirque has been excavated 



