KN'0\VLi:i)Gi:. 



APKII.. 1<>12. 



By G. \V. rYRKE-Li., A.K.C.Sc, F.G.S. 



Tin-; INVAKIAUILITY OK IGNEOUS KOCK 

 ASSOCIATIONS. — An intorostiiiK case of innKiiiatic difTei- 

 entiatioii lias been ilesciibed fri)iii Tripyraniid Mountain, 

 New Hainpsliirc. l>y Professor L. V. Pirsson and \V. X. Rice 

 {Aiiicr. Joiirii. Sci. \\m\-'S\:iy, 1911), which also raises the 

 (jiieslion as to how far certain associations of i(jneons rocl<s 

 are invariable. The igneons complex, which has an ov.il 

 outcrop nieasnrinK two miles by one and a half miles, is 

 intruded into the great granite batholith forming the White 

 Mountains of New Hampshire. It has a laccolithic habit, 

 and a concentric arrangement of rock-types. There is an 

 interrupted marginal zone of gabbro, with norite. followed by 

 an unbroken zone of monzonite, whilst the interior of the mass 

 is occupied by alkali-syenite lumptekite). The complex is 

 accompanied by several dykes of camptonite. and is cemented 

 along the joint-planes by dykes of aplite. The authors 

 explain the phenomena by an hypothesis of successive 

 intrusion with accompanying differentiation. The monzonite 

 was intruded first with the concomitant separation of gabbro 

 and norite on the margin. .\ second intrusion of monzonite 

 then took place, with separation of the interior syenite. 

 Lastly the joints and cracks in the solidified mass were 

 cemented by the final residual product — a quartz-syenite- 

 aplite. This explanation accounts for the arrangement of 

 the rocks within the complex, and. at the same time, for the 

 abrupt transitions and broken angular contacts between the 

 various types. 



The association of types, however, is rather unusual. Thus, 

 alkaline types such as umptekite, monzonite and camptonite, 

 are associated with calcic types, gabbro and norite, in the 

 same complex. This seems to contravene the general rule, 

 established for a great number of occurrences, that the 

 alkaline rocks do not occur in association with the calcic, 

 and raises the point as to w'hether these associations are 

 really invariable. As Professor Pirsson remarks, some 

 petrologists would consider the gabbro and norite as aberrant 

 forms of essexite, the basic member which frequently com- 

 pletes an alkaline series. In that case, he rightly argues, the 

 definition of essexite becomes so broad as to lose all specific 

 value. If the Tripyraniid gabbro and norite are not accepted 

 as such, we are confronted with the alternative of calling 

 similar rocks "gabbro" or "essexite," according to the 

 association in which they are found. 



Accepting the Tripyramid association, however, the great 

 generaliz.ition of the invariable association of certain types of 

 igneous rocks, based on numerous examples throughout the 

 world, is not, we think, to be imperilled by small, isolated and 

 exceptional occurrences such as this. Moreover, we note, 

 from the chemical analyses given, that the norite and gabbro 

 in this suite are considerably richer in alkalies (for a smaller 

 silica content) than the average norite and gabbro, the com- 

 position of which is given in Daly's tables. The difficulty in 

 nomenclature will probably be met by a closer character- 

 ization of igneous types. Thus the norites, a small, restricted, 

 and comparatively rare group, still evidently include different 

 varieties, not as yet expressed in the nomenclature, some of 

 which may be associated with the alkaline, but the majority 

 with the calcic series, 



WATER SUPPLY AND THE DIVINING ROD.— The 

 United States Geological Survey continue to issue their 

 unique series of Water-Suppl>- Papers. The frequency with 

 which a second edition or reprint of many of these publica- 

 cations is demanded is proof of their utility to the public 

 and an example of the excellent economic work that can be 

 carried on b>' a national survey when a broadrninded Govern- 

 ment provides adequate funds for its proper upkeep. One of 

 the latest of this series to achieve a second edition is Paper 

 No. 255, on " Underground Waters for Farm Use," by M. L. 

 Fuller. This paper deals in simple language with every con- 

 ceivable phase of water-supply on farms and cannot fail to be 

 of immense practical value to English-reading farmers every- 



where. Seventeen well-chosen plates and twenty-seven dia- 

 grams help out difTicult places in the text. 



Mr. I^'uller gives an interesting note on his own experience of 

 the divining rod, the simple forked branch of witch-hazel used 

 by the so-called water-witches as an indicator of underground 

 water. We (|uotc his remarks (page 15): "In ex|)eriments 

 with a rod of this type, the writer found that at certain points 

 it seemed to turn downward independent of his will, but more 

 complete tests showed that this down-turning resulted from 

 slight and. until watched for, unconscious muscular action, 

 the effects of which were communicated through the arms and 

 wrists to the rod. No movement of the rod from causes 



outside of the body could be delected The use- 



lessness of the divining rod is indicated by the facts that it 

 may be worked at will by the operator, that he fails to detect 

 strong water-current in tunnels and other channels that afford 

 no surface indications of water, and that his locations in 

 limestone regions where water flows in well-defined channels 

 are no more successful than those dependent on mere guesses. 

 In fact, its operators are successful only in regions in which 

 ground water occurs in a definite sheet in porous material or 

 in more or less clayey deposits, such as pebbly clay or till. 

 No appliance, either mechanical or electrical, has 

 yet been devised that will detect water in places where plain 

 common sense will not show its presence just as well. The 

 only advantage of employing a " water-witch ' .... is 

 that crudely-skilled services are thus occasionally obtained, 

 since the men so employed, if endowed with any natural 

 shrewdness, become through their experience in locating wells 

 better observers of the occurrence and movements of ground 

 water than the average person." 



BRITISH PILLOW-LAVAS.— Two further occurrences. 

 in the Tavistock- Launceston area, and in the Kilbride 

 Peninsula, Mayo, have been added to the already long list 

 of pillow-lavas, which are so well-developed upon several 

 Palaeozoic horizons in Great Britain. 



These rocks usually belong to the peculiar group of the 

 spilites, in which felspars rich in soda form the major part of 

 the rock, w-ith subordinate augite. occasional olivine, and some 

 glassy base. A distinctive micro-structure is shewn by the 

 felspars, which are long, acicular and pointed, with lluidal 

 arrangement, but often also variolitic. The spilites are 

 associated with albite-diabase. minverite, picrite, (juartz- 

 diabase, keratophyre and soda-granite, forming a well- 

 characterised group of rocks named by Dewey and Flett 

 the "spilitic suite." 



The new Devonian occurrences are fully described in a 

 recently-issued Survey Memoir on "The Geology of the 

 country around Tavistock and Launceston." They belong 

 to two horizons, the Upper Devonian and the Lower Culm 

 Measures. Many of them are highly decomposed and 

 rendered schistose by pressure, so much so that it would be 

 more correct to describe them as schalsteins. The intrusive 

 rocks associated with the spilites are quartz-keratophyre. 

 minverite, picrite. and quartz-diabase. 



The Irish pillow-lavas are described by Mr. C. I. Gardiner 

 and Professor S. H. Reynolds, in a paper on "The Ordovician 

 and Silurian Rocks of the Kilbride Peninsula (Mayo) " 

 {Q.J.G.S.. February. 1912), an addition to the important 

 series of papers by these authors on the Palaeozoic Rftcks of 

 the W'est of Ireland, The pillow-lavas are here associated 

 with sediments of .\renig age, and belong to a widespread and 

 extensive series, Spilite and felsite breccias occur proving 

 the presence of an acid magma at no great distance, although 

 no acid lavas have been found in the area. Later on, but 

 almost certainly in .\renig times, one large and many small 

 intrusions of felsite took place. No contemporaneous basic 

 intrusions appear in the area. At an early post-silurian date, 

 however, lime-bostonites and labradorite-porphyrites. which 

 may form part of a later spilitic suite, were intruded. 



Ordovician pillow-lavas are also known in the Girvan- 

 Ballantrae district of Ayrshire, and Megavissey and Mnllion 

 Island in Cornwall, pointing to a spilitic petrogiaphical 

 province of enormous extent covering the Western British 

 area in Ordovician limes. 



