Academy of Sciences] 

 No. 5] 



POWELL'S SURVEY 



81 



essentially speculative nature of geological science; for various matters which are set forth as 

 "the principle facts in regard to the laccolites" are in reality speculative inferences that far 

 transcend the reach of observation ; indeed, they are nothing more than the thriving outgrowths 

 of certain venturesome speculations of a century or more ago, which have been since then 

 accredited and accepted as the veritable counterparts of invisible, unobservable facts in the 

 vanished past. The production of the laccolithic rock masses by cooling from a state of fusion ; 

 their ascent, while fused, from a deep source by unseen conduits; the upheaval of the covering 

 strata as a result of the ascent; and the later removal of more or less of the covering strata by 

 erosion — all these and various similar matters are evidently not visible facts of observation 

 but only accepted concepts of speculation, yet so successfully fortified that they are properly 

 taken, along with visible facts, as realities to be explained by other speculations. 



Only one of these speculative conclusions was regarded as in need of special inquiry for 

 its justification; namely, that the laccolithic masses are really subterranean intrusions which 

 blistered up the strata above them, and not ancient volcanic masses erupted upon the earlier- 

 deposited, underlying strata and buried under the later-deposited overlying strata. To this 

 inquiry, "the answer is not difficult." The laccoliths are subterranean intrusions; for this 

 conclusion, which after it is reached is treated with entire geological propriety as a "fact," is 

 supported to the point of demonstration by the standard scientific method of first deducing 



rwiKm^imwms^-- 



Fig. 10— The Western face of the Mornne lacolith 



contrasted groups of consequences from the two suggested possibilities of intrusion and extru- 

 sion, and then impartially confronting the deduced consequences with appropriate items of 

 observation; a method of proof that is truly simple enough; so simple that some readers may 

 think it hardly worth mentioning here; yet it is doubtful whether the question at issue — the 

 explanation of buried igneous masses as subsequent intrusions or contemporaneous extru- 

 sions — had previously been treated in this way as fully by anyone else. Indeed, the failure 

 of some of Gilbert's predecessors to apply this simple method in the investigation of buried 

 igneous masses elsewhere delayed them in the attainment of safe interpretations. 



THE BASE OF THE LACCOLITHS 



A metaphysically minded reader may object to the use above of such words as demonstra- 

 tion and proof in connection with a method of inquiry that leads, as geologists very well know, 

 to nothing more than a high order of probability, and even to that only on the pragmatic assump- 

 tion that the present order of nature has endured all through the geological past ; but with this 

 sort of objection to the interpretation of laccoliths and other geological phenomena we are not 

 here concerned. More pertinent are the doubts that were expressed by certain distant scien- 

 tists 30 or 40 years ago as to the geological validity of Gilbert's conclusions. For example, 

 Reyer, of Vienna, apparently more guided by his own prepossessions than by Gilbert's evidence, 

 contended that the Henry Mountains laccoliths must really be buried volcanoes and not sub- 

 terranean intrusions. 2 Neumayr, deservedly regarded as one of the leading geologists of his 

 time, inclined to the same view, but in deference to Gilbert's explicit statement of the reasons 



" Theoretische Geologie, 1888, 135, 136. 



