HA R D WI CKE 'S S CIE NCE -GOS SIP. 



69 



making meadow-rue to flourish on solid limestone ; or 

 stirring the bile of plodding botanographers by defining 

 habitats which turn out to be several miles distant from 

 the spots where specimens were actually observed. 

 Slips of the pen seem innocent and harmless, but un- 

 luckily they retain vitality enough to pass from book 

 to book, gathering authority as they go, long after the 

 possibility of taking the careless writer to task has 

 passed away. If not shot at first sight, such lapses 

 are hard to kill.— >.f. JVallcr White, Clifton. 



GEOLOGY, &c. 



"Geology and Mining Industry of Lead- 

 viLLE, Colorado, ^VITH Atlas," by Samuel Frank- 

 lin Emmons (Washington : Government Printing 

 Office). This voluminous and well got-up volume 

 (which forms the eleventh vol. of the " Mono- 

 graphs of the United States Geological Survey"), 

 sustains the high reputation earned by the preceding 

 volumes. The author (Samuel Franklin Emmons) 

 opens with a topographical description of Leadville ; 

 its position, discovery, and devejopment. The narra- 

 tive of the discovery of gold here, and the subsequent 

 rush to the gold-fields is most interesting, and the 

 stories of the fortunes made there, make a poor 

 editor's pockets feel very light. Mr. Emmons 

 describes the microscopical and chemical composition 

 of various rocks and minerals associated with the 

 district he has so well studied. The volume is 

 artistically illustrated throughout, and a very useful 

 geological atlas is added. One of the most important 

 chapters is that on the Composition of Vein Materials 

 and the Origin of Ores. This important contribution 

 to mineralogical and geological science extends to 

 nearly 800 pp. quarto. We cannot but compare the 

 generosity with which the United States Government 

 send out gratuitously to English scientific journals, 

 with the beggarly conduct of our English Stationery 

 Office, which does not even send out for review to the 

 English Press any of the scientific volumes, brought 

 out at such vast expense either by our own Geological 



Survey, or of the " Challenger " Expedition. 



Misconceptions regarding the Evidence 



WHICH WE OUGHT TO EXPECT OF FORMER GlACIAL 

 Periods.— Dr. James Croll, F.R.S., recently read a 

 paper on the above subject before the Geological So- 

 ciety. He said the imperfection of the geological record 

 is greater than is usually believed. Not only are the 

 records of ancient glacial conditions imperfect, but this 

 follows from the principles of geology. The evidence of 

 glaciation is to be found chiefly on land-surfaces, and 

 the ancient land-surfaces have not, as a rule, been 

 preserved. Practically the several formations consist 

 of old sea-bottoms, formed out of material derived 

 from the degradation of old land-surfaces. The 

 exceptions are trifling, such as the under-layers of 

 coal-seams, and dirt-beds like those at Portland. The 



transformation of an old land-surface into a sea- 

 bottom will probably obliterate every trace of 

 glaciation ; even the stones would be deprived of 

 their ice-markings ; the preservation of Boulder-clay, 

 as such, would be exceptional. The absence of large 

 erratic blocks in the stratified beds may indicate a 

 period of extreme glaciation, or one absolutely free 

 from ice. The more complete the glaciation, the less- 

 probability of the ice-sheet containing any blocks,, 

 since the rocks would be covered up. Because there 

 are no large boulders m the strata of Greenland or 

 Spitzbergen, Nordenskjold maintains that there were- 

 no glacial conditions there down to the termination of 

 the Miocene period. The author maintained that 

 glaciation is the normal condition of polar regions, 

 and if these at any time were free from ice, it could' 

 only arise from exceptional circumstances, such as a 

 peculiar distribution of land and water. It was 

 extremely improbable that such a state of things could 

 have prevailed during the whole of the long period 

 from the Silurian to the close of the Tertiary. A 

 million years hence it would be difficult to find any 

 trace of what we now call the glacial epoch ; though 

 if the stratified rocks of the earth's crust consisted of 

 old land-surfaces, instead of old sea-bottoms, traces 

 of many glacial periods might be detected. The 

 present land-surface will be entirely destroyed in order 

 to form the future sea-bottom. It is only those 

 objects which lie in existing sea-bottoms which will 

 remain as monuments of the Post-tertiary glacial' 

 epoch. Is it, then, probable that the geologist of the 

 future will find in the rocks formed out of the non- 

 existing sea-bottom more evidence of a glacial epocli 

 during Post-tertiary times than we now do of one, say, 

 during the Miocene, Eocene, or Permian period "r 

 Palaeontology can afford but little reliable information 

 as to the existence of former glacial periods. In the 

 discussion which followed, the president considered 

 that the author underrated the amount of old subaerial 

 surface. Many freshwater deposits, such as the 

 Wealden, were fluviatile and hence subaerial. And if 

 glacial materials had then been in the neighbourhood 

 they would have been preserved. Professor Prestwich 

 thought if glacial periods had formerly existed we 

 should not be dependent only on land-surfaces, but 

 the molluscan fauna of the Arctic seas, and the glacial 

 debris and boulders spread over the bed of those seas 

 would bear evidence of analogous conditions. We 

 had in India and Australia some evidence in favour of 

 such conditions in Permian and Carboniferous times, 

 but these even were not yet fully established ; and as 

 to Eocene and Miocene times, he knew of no evidence. 

 Dr. Evans said, whether the theory were true or not, 

 glacial conditions must in all probability have pre- 

 vailed in some parts of the globe during all periods, 

 though probably not always in the existing centres of 

 glaciation ; and of these former glacial conditions 

 some evidence was already forthcoming. The alleged 

 misconceptions did not seem to him to exist, unless to a 



