6o6 



NATURE 



{Oct. 28, 1880 



Metamorphic Rocks, Ireland 



There appears to be confusion as to the times when meta- 

 morphic action occurred among the Irish rocks ; my experience 

 would point to the following : — 



In the Carnsore district, South-East Wexford, there are meta- 

 morphic rocks for a long time supposed to be of Lower or 

 Cambro-Silurian age; I however proved that they were up- 

 turned, contorted, metamorphosed, and denuded, prior to the 

 overlying fossiliferous Cambro-Silurian rocks being deposited, 

 and for the reasons given in the Geological Survey Memoir it 

 is probable these metamorphic rocks are of Cambrian age. 



In the hills north of Pomeroy, Co. Tyrone, there are meta- 

 morphic rocks, which were upturned, contorted, metamorphosed, 

 and denuded, prior to the overlying fossiliferous "Pomeroy 

 rocks " having been deposited. The fossils in the latter would 

 point to their being Carabro-Silurians ; consequently the meta- 

 morphic rocks are older, and for reasons given in a paper read 

 before the Koyal Irish Academy I believe they are the equiva- 

 lents of the "great micalite series," West Galvvay, or the 

 equivalents of the Arenig group of Wales. Thai is either 

 Upper Cambrian, or Passage bids between the Cambro-Silurian 

 and Cambrian. 



In Erris, North-West Mayo, there is a tract of excessively 

 metamorphosed rocks, supposed by Griffith to be older than 

 the associated altered Cambro-SUurians, and this opinion is 

 shared in by Mr. McHenry, who more recently examined them. 



From the above it is evident that there was a period of intense 

 metainoyp/iosis prior to the Cambro-SiluriaH age. 



The Cambrian (Arenig group?) and Cambro-Silurian of 

 Galway and South West Mayo must, in part, have been altered 

 prior to the deposition of the Upper Silurians on them ; while 

 the general metamorphism of the South-East Ireland Cambro- 

 Silurians, vjhieh was quite irrespective of the intrusion of the 

 Leinster granite, was probably at about the same time. If the 

 Coraeragh Mountain rocks are Glengariiif grits, i.e. Silurians, 

 the age of the metamorphic action is evident, as in Waterford 

 these rocks underlie those of the Comeragh Mountains. In 

 addition to the general metamorphism in the recks of South- 

 East Ireland, there was also a local and secondary action in 

 connection with the protrusion of certain granitic rocks. 



The testimony of the West Galway and South-West Mayo 

 rocks alone, however, would prove a period of intense meta- 

 morphic aelion at the close of Cambro-Sitiirian time. 



lu South-West Mayo, as proved by Mr. Symes and myself 

 (Maps and Memoirs of the Geol. Survey], there is a considerable 

 area of metamorpho^ed Upper Silurian rocks, which prove 

 another period of intense metamorphic action siihseqitent to the 

 dawn of Upper Silurian times. The secondary metamorphism 

 previously mentioned in South-East Ireland may also be of this 

 age, as the granitic rocks allied with the metamorphic, in both 

 areas, are very similar. 



Tims there are records of at least three periods of intense 

 metamorphic action, and probably there were two others sub- 

 sequently — one in the Triassic and another in the Miocene time 

 — to account respectively for the metamorphic rocks in the 

 neighbourhood of the Mourne granite, Co. Down, and those 

 associated with the granitic rock near Portrush, Co. Antrim. 



Formerly, as mentioned by me in the " Geology of Ireland," 

 the period of greatest metamorphism \\as considered to have 

 been at the close of the Cambro-Silm'ian time ; now, however, 

 more recent research has taught us that metamorphic rocks, 

 formerly supposed to be Cambro-Silurians, are Cambrians ; so 

 it seems possible the metamorphic action prior to Cambro- 

 Silurian time may have been greater than that subsequent to it. 

 Ovoca, Ireland G. H. Ki.NAHAN 



The Number of Known Species of Hemiptera- 

 Heteroptera 



As Mr. Pascoe, in his very "handy book of reference" for 

 zoological classification, says of the Hemiptera-Heteroptera that 

 " in round numbers there maybe about 10,000 species in this 

 sub-order," I am induced to give my census of the group. 



On completing, about a year ago, MS. lists of the families 

 which Stil unfortunately did not live to include in his " Enume- 

 ratio Ilemipterorum," I was tempted to try and ascertain the 

 total number of species that had been described. This I found 

 to be about 7,800 (the actual number arrived at is 7,780). Of 

 these, 7,445 belong to the Geocorisa; or Gymnocerata (mostly 

 terrestrial bugs, but including four families which inhabit the 



surface of water), and 334 to the Hydrocorisa; or Cryptocerata 

 (almost all aquatic species). Of the Geocorisa: 1,503 are Euro- 

 pean, 3,248 are natives of the rest of the Old World, and 2,694 

 are American ; of the HydrocorisK the corresponding numbers 

 are 95, 120, and 119. The largest family of the Geocorisas in 

 Europe is the Capsidx with 500 species, as against 134 and 312 

 in the rest of the Old World and America respectively. Amongst 

 tlie liydrocoriste the family Corixida' is most numerous in species, 

 the numbers being : for Europe 72, the rest of the Old World 

 1 7, and America 34. But as these two families contain many 

 inconspicuous species, and species having a strong resemblance 

 inter se, and as Europe has been (naturally) more thoroughly 

 investigated than the other regions, it is likely that many extra- 

 European species of these families remain yet to be discovered. 



Of what may be the actual number of sp:cies of Hemiptera- 

 Ileleroptera existing it is difficult to form an estimate. It is 

 only of late years that much attention (comparatively) has been 

 directed to the order, and from the number of new species sent 

 home by the few collectors who condescend to collect bugs, 

 it is evident that very great additions to the list will in course of 

 time be made. Even within the last twenty years the list has 

 been more than doubled, as in A. Dohrn's catalogue, published in 

 1S59, only 3,627 are mentioned. 



Of the sub-order Homoptera it would be rather difficult to 

 make a census. In Dohrn's catalogue somewhere about 3,000 

 species are catalogued — a number not very far short of that of 

 the Ileteroptera. In the British and European lists the number 

 of Homoptera is about two-thirds that of the Ileteroptera. 



Perth, October 19 F. Buchanan White 



On the Classification of Rivers 



It has often occurred to me that a convenient classification of 

 rivers might be obtained by arranging them according to their 

 "water-discharge." Such a classification would not only indicate 

 the relative position of one river to another in a descending 

 scale, but would enable a rough estimate to be borne in the 

 memoiy of the amount of water any particular river may 

 discharge. 



I therefore venture to suggest the following arrangement : and 

 have given below the names of seventeen 'rivers, the discharges 

 of which I have obtained from various sources, for which I 

 would refer the reader to Nature, vol. xxii. p. 4S6. 

 Discharge of Cubic Feet per second 



Woodlane, Falmouth, October 19 



H. B. GUPPY 



Yuccas under Cultivation 

 In Nature, vol. xxi. p. 315, in the report of the Proceedings 

 of the I.innean Society, it is stated that " the yuccas fruit rarely 

 under cultivation, the large white pendulous flowers being in the 

 wild plant fertilised by a moth of the genus Pronuba." The 

 yucca has been introduced and is very abundant in this colony, 

 especially round Noumea. It fruits freely ; in fact I rarely see 

 a plant in which many, if not most, of the flowers do not pro- 

 duce seed-pods. In my own garden they seem to be fertilised 

 by the common bee, of which 1 have a hive, others being in the 

 neighbourhood. If I remember rightly, Pronuba is a genus of 

 large moths having yellow underwings. We have a species 

 identical with, or closely resembling, an old Ceylon friend, but 

 it is rare ; still it does exist here, and may assist in the fertilisa. 



