MUSEUM OF COMPARATIVE ZOOLOGY. Ill 



these facts, which were first pointed out by Messrs. Foster and Whit- 

 ney', and later by Mr. Marvine, is that given by them, that the traps are 

 lava flows, and that they were successively laid down upon one another, 

 or covered by sandstone and conglomerate. They are seen to have the 

 same characters, except so far as they have undergone secondary changes, 

 that modern basaltic lavas have. These old basalts have been denomi- 

 nated melaphyrs and diabases by Prof. Pumpelly, to whom lithologists 

 are indebted for their knowledge of their microscopic characters. Wliile 

 we would use the terms that Prof. Pumpelly has, we object to the ap- 

 plication he has made of them. Many of his diabases we should call 

 melaphyrs, and many of his melaphyrs we should class as diabases. 

 We of course differ in our definitions of these terms, for while he would 

 regard melaphyr and diabase as distinct rock species, we hold that they 

 are only altered forms of basalt. The greenstone ridge back of the 

 Cliff and Phoenix mines we regard as an excellent example of diabase 

 (791, 792), with which we class all the heavy-bedded crystalline traps 

 of that region, while the thin-bedded scoriaceous or amygdaloidal highly 

 altered traps we class as melaphyr, but in the majority of cases Prof. 

 Pumpelly regards them as the reverse. The diabases are jarely if ever 

 mined, the melaphyrs frequently. 



In a recent paper " On the Carboniferous Volcanic Rocks of the 

 Basin of the Firth of Forth — their Structure in the Field and under 

 the Microscope," * Prof Geikie points out the difference microscopically 

 between lava flows and intrusive masses, and evidently thinks that 

 they can always be as readily separated in the cabinet, microscopically, 

 as they can be in the field. While it is doubtless true that this sepa- 

 ration can be made in the rocks studied by Prof. Geikie, his distinc- 

 tions fail iu the Lake Superior district. The difference in structure 

 pointed out by him seems to be entirely owing to the rate of motion 

 and pressure at the time of crystallization, and the rapidity with which 

 the lava solidified. When lava flows in thin sheets, or, if we con- 

 fine our examination to the upper portion of a thick sheet, we find 

 cliaracters that readily distinguish the sheets from the dikes ; but when 

 we come to study the middle and lower portions of the thick sheets, 

 where there was little or no motion combined with the pressure of the 

 overlying mass, the rock is imdistinguishable from rock of the same 

 composition in dikes ; and the diagnostic characters given by Prof 

 Geikie would assign it to an intrusive rock, not to an overflow. In the 

 before-mentioned Emerson adit at the Copper Falls mine, some mela- 



* Trans. Royal Soc. of Edinburgh, XXIX. 437-518. 



