362 



• KNOWLEDGE . 



[Oct. 31, 1884. 



soft hat, large mackintosh, and a good large bag to carry 

 his specimens. I use a gamekeeper's bag, although it 

 sometimes attracts undesirable attentions. Two hammers 

 are desirable— a heavy one, weighing about 3 lb., and pro- 

 vided with a long ash handle to cope with the massive, tough, 

 and often rounded rocks, and a light i lb. trimming hammer, 

 to reduce specimens to shape in the field; geological hammers 

 are square in the iace, and should be made of good steel. 

 A cold chisel (as the solid iron chii)ping tool is called) is 

 useful for detaching fossils from the rocks, while a magni- 

 fying glass (Browning's platyscopic lens is the best) is very 

 necessary to examine minerals, &,c. Of other requisites, 

 the coloured map of the district, executed by the Geological 

 Survey is most useful ; Sheet 75 (in four parts or " Quarter 

 Sheets," price 3s. each) includes the district in which we 

 are more directly interested. A pocket-compass is very 

 useful in this rather lonely region, where it is quite the 

 exception to find a labourer or field-hand who can speak 

 English ; the compass may form part of a clinometer, an 

 instrument with which we can measure the dip or slant of 

 any set of rocks. A tape-measure is handy, when it is 

 desired to measure the thickness of the strata, or the depth 

 of the rock-sections exhibited in clitfs, cuttings, &c. A 

 note-book and pencil ought certainly to be carried, in order 

 to note down, on the spot, the particulars relative to each 

 quarry or other exposure. All these articles may be procured 

 from Mr. Gregory, of Charlotte-street, Fitzroy -square. 



How to Collect Hocks. — Every specimen collected ought 

 to be capable of identification, as to the exact locality from 

 which it came, at any futuie time. Such particulars may 

 be noted in pencil upon the paper in which the rock, 

 fossil, or mineral is wrapped, or upon a slip of paper placed 

 with the specimen ; liut a better plan is to carry a series of 

 gummed and perforated numbers, and to attach one of these 

 to each specimen as it is secured, copying the number in the 

 note-book, and adding the locality, date, itc. Each specimen 

 sliould be wrapped up separately (best in thin, tough brown 

 p'per) so that it may not grate against and scratch its 

 lellows. Remember that small angular bits of rock are of 

 little use; try to obtain neat, oblong specimens — 3 inches long 

 by 2 broad, and 1 inch thick is a useful size — and let them 

 be of fresh un weathered stutl" taken from the middle of a 

 block, and as far below the surface as possible. 



There is only one book which treats specially of the 

 rocks of Carnarvonshire — Ramsay's " Geology of North 

 Wales," Stanford, 21s. ; but it is a splenflid work of 600 

 pages and many illustrations. The author. Sir A 0. 

 Ramsay, late head of the Geological Survey, knows the 

 ground well, having either mapped, or aided in mapping, a 

 large portion of the district : and although we may not 

 always agree with his theories, yet his facts are numerous 

 and reliable. 



Rocks of the Lleyn Peninsula. — The projecting mass 

 which forms the northern horn of Cardigan Bay and the 

 western half of Carnarvonshire, is known as the Lleyn 

 Peninsula. Viewed broadly it consists of certain extremely 

 hard rocka, usually called igneous, because they are supposed 

 to have been at one time liquefied by the action of heat, 

 surrounded by softer sedimentary strata which mantle 

 round the unstratified igneous bases, forming an undu- 

 lating plain. The geological age of all these rocks is indi- 

 cated in the following table, the oldest or first-formed being 

 ■nlaced at the bottom. 



Bala Beds, 1,000 feet? ( 



Arenig Slates, 300 feet ? ^ 



Tremad:c Slates, to 200 feet "^ 



Lingu'a Flags, 1,200 to 2,000 feet ) 



Schistose Rocks, rorphyritic ) PreCambgias 



Pelstcces, &c., thickness unknown ) Form.4t;on. 



LoWEK 

 SiLCKLAN 



Formation. 

 Cambria.v 

 Formation. 



Pre-Cambrian Rocks. — A fierce conflict has long raged, 

 and is still raging, as to whether there are any rocks in 

 Wales of older date than the Cambrian strata. Strange to 

 say, the dispute has resolved itself into one between 

 " olficial " and "non-official" geologists. The new chief of 

 the Geological Survey — Professor A. Geikie — upholds the 

 views of his predecessor (Sir A. C. Ramsay), and is sup- 

 ported by his colleagues in arguing against the existence of 

 such rocks ; while Dr. Hicks, with Professors Hughes, 

 Bonney, Lapworth, Blake, Callaway, and others, ardently 

 assert that at various points — as around St. David's, in 

 South Wales, and at several spots in North Wales, and 

 Anglesea — very ancient strata do exist, comparable in age 

 with the Archsean Gneiss of the Hebrides, and the 

 Laurentian Rocks of Canada, the very bottom beds of the 

 earth's crust. 



But, it may be asked, if the officers of the Geological 

 Survey deny the existence of pre-Cambrian rocks in Wales, 

 what age do they assign to the rock-masses whose 

 claim to that high antiquity was first upheld by 

 Dr. Hicks? To this the reply would doubtless be, 

 that each case, each locality where such rocks are said 

 to occur, must be dealt with separately, for the explanation 

 that might be applicable to one example would very pro- 

 bably not answer elsewhere. Still, the Survey views of the 

 so-called pre-Cambrian rocks may be .summed up under two 

 heads :— («) They are altered Cambrian strati Cambrian 

 rocks which have been so baked and pressed, and altered 

 by chemical action at some former time when they were 

 deep down, far below the surface, that their appearance has 

 been quite changed, and any fossils which they may once 

 have contained have been destroyed ; or (h) they are intru- 

 sive, igneous rocks which have invaded, while in a liquid 

 state, the strata lying above them. The first explanation 

 would be used when the " pre-Cambrians " are of a schifl- 

 tose or slaty nature ; the latter, when they are felsitic or 

 granitic in appearance. Let us examine such of the rocks 

 that lie within an easy drive of Pwllheli or of Criccieth, as 

 would probably be classed by the " new school " with their 

 pre-Cambrian beds. 



The little town of Nevin lies seven miles north-west of 

 Pwllheli, with which it is proposed to connect it by a rail- 

 way, although it is difficult to see where the traffic to pay 

 dividends on such a line is to come from. The mail-car 

 affords a ready means of transit between the two towns, 

 leaving Pwllheli at eight in the morning and returning 

 from Nevin between four and five in the afternoon ; 

 or a trap may be hired from the Madryn Arms 

 at a very reasonable rate. Reaching Nevin the geo- 

 logist finds himself on the shore of Carnarvon Bay, 

 and overlooking the splendid natural harbour of Porth- 

 Dinlleyn. From the western side of this bay a^ rugged 

 promontory juts into the sea. The rocks of this head- 

 land are marked as "serpentine" on the Survey Map ; 

 but Prof. Bonney has lately shown them to be compact 

 diabases, very probably ancient lavas. In such a case as 

 this we must remember that the Survey Map was executed 

 thirty or forty years ago, when the modem method of 

 investigation of rock structure by means of the microscope 

 was unknown. In the case of this Porth-dinlleyn rock it 

 is chiefly by the study of slices cut to the one-thousandth 

 of an inch in thickness that we are able to ascertain its 

 real nature. 



The beds traversed by these lavas are volcanic breccias, 

 consisting of dust ejected from some vents then in activity, 

 but also containing innumerable angular blocks of all sizes, 

 all non-compacted with a greenish, patchy-looking rock. 

 South of Porth-Dinlleyn, green and grey compact schistose 

 rocks, traversed by numerous veins of white quartz, extend 



