MUSEUM OF COMPARATIVE ZOOLOGY. 73 



of geologists in every quarter of the world docs not establish any- 

 thing, if that iigrcenient is based on the same tlieories and methods, 

 unless the theories and methods are correct. The evidence is clear 

 enough in New England and at Lake Superior that the theory is un- 

 proved, and the methods and observations incoiTect or superficial. It 

 seems to us that a striking conunentary on the value of litliological 

 characters is aftbrded in the Marquette district by the letters and the 

 comments upon them given on pages 657 to GGO, inclusive, of the third 

 volume of the Geology of Wisconsin. 



We frequently are informed that microscopic analysis will make up 

 for all deficiencies in field work ; that one can tell in this way whether 

 a granite or any other rock is eruptive or metamorphic ; that Formation 

 XX. is to be distinguished in this way from the Laurentian granite, 

 etc., etc.* 



The study of rocks microscopically enables us to investigate their 

 structure, constitution, and alterations, — the structure of their constitu- 

 ents and the various relations that these bear to one another, as well as 

 the order of their formation. It gives us the internal history of a rock 

 more or less complete, — a thing that no other known method will do. 

 It enables us to tell the species, arrange, and generally to classify our 

 rocks. With the exception of such as are greatly altered, it enables us 

 to distinguish the fragmental from the non-fragmental foi*ms. When we 

 are familiar with the microscopic characters of unchanged sedimentary 

 rocks and of unaltered volcanic ones, they having been known to be 

 such from field evidence, we have a basis for recognizing rocks, con- 

 cerning whose field relations we know nothing, as belonging to one or 

 the other of these classes, the same as by the unaided eye we recognize 

 a coarse granite or conglomerate. This of course only applies to rocks 

 concerning whose nature and origin there is no dispute ; or to those 

 which under the microscope show such undoubted evidence of their 

 origin that it cannot be rejected. If a rock is clearly seen under the 

 microscope to be made up completely of fragments, no one would doubt 

 that it was a fragmental rock, even if it looked to the iniaided eye as 

 though it were a non-fragmental one. In like manner, when a rock has 

 the microscopic characters of an eruptive one, we feel that it is right to 

 regard it as such, even if it was supposed (not proved) to be a sedimen- 

 tary one by the collector in the field. Both of these are of frequent 

 occurrence in the work of a lithologist. Take now the great interme- 

 diate class of rocks, those that are so altered that their original charac- 



• Geol. of Wise, II. 73 ; III. 194, 255. 



