HARDWICKE'S SCIENCE-GOSSIP. 



219 



it, but the older editions of Babington's Manual 

 contain it in the form of a concise key, and many 

 secondhand " Floras " on the same plan may be had. 

 As far as I am aware, there is no artificial key in 

 English on a plan which may be fairly called 

 " thorough." Authors undertake to write one, but 

 have not the courage to do it. They have a secret 

 liking for the natural system, and must give one a 

 key to the natural orders and the genera, as well as 

 the species ; the result is, they make the key longer 

 than necessary, by arranging their alternatives to 

 carefully exclude first the orders, then the genera 

 to which the plant does not belong, before coming to 

 the question of species. Even Linnaeus must group 

 the species into natural genera in his artificial 

 system. Why not go straight to the point and 

 enable one to ascertain the species as quickly as 

 possible ? The species ascertained, it is merely a 

 question of referring to a dictionary or an index, to 

 learn the characteristics and affinities of the genus and 

 natural order. Some "Floras" seem to be written 

 as though the author never thought of their being 

 used to determine the species. If a "Flora" be 

 merely intended to give one a general idea of the 

 plant-life of a particular district this method is not 

 objectionable, but if it be intended for practical and 

 field use every assistance in the way of keys, indexes, 

 and other helps should be given in order that no 

 time may be lost. Some seem to consider it deroga- 

 tory to their dignity to discover the name of a plant 

 by any artificial means ; but " Floras," arranged on 

 the most natural system, and in fact most suitable for 

 teaching the affinities of plants already well-known, 

 are by no means necessarily the best for practical use, 

 any more than a scientific classification of words, 

 according to their significations, would enable one to 

 find out the correct spelling of any particular word, 

 as easily as the alphabetical arrangement of a 

 dictionary. B. Hobson. 



A NEW DIATOMACEOUS DEPOSIT IN 

 NORTH WALES. 



By W. F. Lowe, Assoc. R.S.M., F.C.S., F.I.C. 



BUT few diatomaceous deposits being known to 

 exist in Great Britain, and the interest attach- 

 ing to them being so great, both to the geologist and 

 microscopist, it has occurred to me that the following 

 account of one I was fortunate enough to discover in 

 Merionethshire in 1879 might be of sufficient im- 

 portance to warrant its publication. Llyn Arenig 

 Bach, the lake in which the deposit occurs, is 

 situated on the east side of the Mountain Arenig Bach, 

 and is, judging roughly, 1200 to 1500 feet above the 

 level of the sea, the mountain being 2264 feet high, 

 and the lake being two-thirds the distance up the 

 mountain. It lies about seven miles to the south 

 of Yspytty-Evan, and midway between Bala and 



Festiniog, being nine miles from either place, and 

 one mile from the high road from Bala to Festiniog. 



The lake is a quarter of a mile long by about 

 three hundred yards broad at its widest end, and 

 is roughly wedge shaped. On the west side of 

 it a steep wall of volcanic ash rises abruptly from 

 the lake almost to the top of the mountain, and on 

 the south side the ground is high and slopes steeply 

 to the lake, while on the east side the ground is 

 lower and slopes more gently, forming a low ridge 

 (believed to be a moraine) beyond which the 

 mountain slopes steeply down in a large valley which 

 runs up towards Yspytty-Evan. 



The stream from the lake flows out at the north- 

 east end of the lake, and at the time of my visit a deep 

 trench had been cut at this point, so as to] drain 

 the lake and lower its level some twenty feet.* The 

 lake is of glacial origin, apparently having been 

 formed by a moraine on the east side damming in 

 the water. 



The rocks around the lake are of igneous origin ; 

 they consist of volcanic ash, and belong to a series 

 of volcanic rocks of the lower Silurian age, which 

 were ejected during the deposition of the Llandeilo 

 Flags. 



The edge of the diatomaceous deposit lies at least 

 ten feet below the original level of the lake, and it 

 could not have been seen if the lake had not been 

 partially drained. 



At the south end of the lake, a small stream running 

 in has brought down large quantities of black peat, 

 and deposited it irregularly over the white earth, the 

 greatest thickness of peat being at the point where 

 the stream comes into the lake, its thickness here 

 being equal to that of the diatomaceous earth, i.e. one 

 foot. Very little peat covers the deposit at the north 

 end of the lake, and some large patches of the white 

 earth are clearly visible in the shallow water. 



Subsequent to the partial drainage of the lake, 

 already referred to, the stream had cut] for^itself a 

 channel of many yards in length through both peat 

 and earth. 



On the sides of this channel sections are exposed, 

 showing very clearly the strongly-marked line of 

 demarcation which sharply divides the two deposits, 

 there being no indication of a gradual transition from 

 the white colour of the earth to that of the black peat. 

 Underlying the deposit was a bed of angular pebbles, 

 forming a coarse kind of gravel, not apparently of 

 much thickness, and composed of fragments of the 

 neighbouring rocks, no foreign or erratic blocks being 

 seen either in the gravel or near the rocks. 



The earth is white in colour and slightly sandy in 

 texture, somewhat resembling a china clay in appear- 

 ance, but microscopical examination shows" it to 



* I have since learned that this had been done in order to lay 

 a pipe to supply the town of Bala with water, and that the lake 

 has now had its level raised, so that the diatomaceous deposit 

 is more than ten feet under water. 



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