i;8 



NA TURE 



{Dec. 25, 1879 



"nless it possessed them. But these characteristic struc- 

 ures were vastly different from the simple anticlinal 

 tilting which the earlier speculators had believed to be 

 the typical arrangement of the beds in a mountain chain. 

 It was found that the rocks had been folded into a num- 

 ber of very sharp troughs and arches whose axes ran 

 roughly parallel to the trend of the chain. The radii of 

 some of the curves were measured by miles, while in 

 other cases the beds had been puckered up into minute 

 and complicated convolutions. Frequently the arches 

 had been canted over, and inversion of the beds had 

 been produced. Slaty cleavage had been largely deve- 

 loped, the planes of cleavage having the same general 

 bearing as the axes of the range. Faulting had taken 

 place on a large scale, and the rocks were often jammed 

 and mashed together till a state of confusion that defied 

 description had been produced. 



No single thrust acting vertically upwards could have 

 brought about such results as the repeated folding, the 

 inversion, the cleavage, and the smashing ; but every- 

 thing pointed to powerful pressure acting in a horizontal 

 direction which had wrinkled up a vast thickness of strata 

 into mighty folds, and sometimes jammed them together 

 till they became little better than a mash of shattered 

 and ruined rock. The crystalline core was in some cases 

 nothing more than the result of intense metamorphism ; 

 and where it was intrusive, there was every reason to think 

 that the molten or pasty rock had been driven up through 

 fissures by the squeezing which the rocks had undergone; 

 in fact, so far from the crystalline centre being the cause 

 of the upheaval, its presence was only one of the results 

 which almost necessarily followed from the way in which 

 that upheaval had been brought about. 



All the facts then seemed to show that mountain chains 

 had not been uplifted by a force acting vertically upwards, 

 but had been ridged up by a squeezing force acting hori- 

 zontally on a very thick mass of strata. 



That denudation carved into shape the mass as it rose 

 was soon realised, but we are here concerned only with 

 the early stages in the genesis of a mountain chain. 



Now one point of great interest in the geology of the 

 Henry Mountains, is that they seem at first sight to form 

 a striking exception to the law of arrangement, perhaps 

 we might more properly say disarrangement, which 

 prevails so widely in mountain chains. They might also, 

 to a casual observer, seem to supply a case where the 

 structure assigned by the earlier geologists to mountain 

 ranges, and which has been looked for in vain so often, 

 does really exist. 



Careful investigation, however, shows that neither of 

 these suppositions would be true. 



The structure of the Henry Mountains is simple when 

 compared with the complicated foldings and disturbances 

 so characteristic of mountain chains. In the case of each 

 of these mountains the strata are arranged in dome- 

 shaped fashion dipping outwards in all directions from the 

 centre. The strata of the plateau from which they rise 

 are all but horizontal ; around the base of each mountain 

 the beds bend up and "rise, slowly at first, but with 

 steadily increasing dip, till an angle of 45° is reached. 

 The dip then steadily diminishes to the centre, where it is 

 nothing." In some cases the beds slope away from a 

 single centre, in others a great arch is made up by the 

 confluence of a number of smaller domes. 



Widely different as this arrangement is from the com- 

 plicated contortion and disturbance usually met with in 

 mountain chains, the Henry Mountains furnish no excep- 

 tion to the broad generalisation that mountain chains 

 always exhibit intense convolution and smashing of their 

 rocks. For the Henry Mountains are in no sense a moun- 

 tain range. They are a group of peaks, each of which is 

 an isolated individual ; they show little or no tendency 

 towards a linear arrangement ; " they would prove per- 

 fectly intractable in the hands of those geologists who 



draw parallel lines through groups of volcanic vents by 

 way of showing their trend. They are as perfectly 

 heterotactous as they could be made by artificial arrange- 

 ment." 



In the case of several of the Henry Mountains the 

 centre of the hill is seen to be occupied by a core of 

 intrusive trachyte, from which intrusive sheets and dykes 

 are given off. Reasoning from analogy Mr. Gilbert 

 believes that in those cases where no such core can be 

 seen, there still is one present under ground, but as yet 

 uncovered by denudation. The upper surface of these 

 cores is arched, and seems to run parallel to the bedding 

 of the overlying rocks. It certainly looks as if we had 

 here a case when strata originally horizontal had been 

 bent up into a dome by the injection from below of a 

 mass of molten rock. And this is the explanation 

 adopted by Mr. Gilbert, but his views differ widely from 

 those which the earlier geologists would have maintained 

 had they been acquainted with these mountains. The 

 earlier speculators gave to their intrusive masses a 

 wedge-shaped form, representing them as broadening 

 downwards and extending to the lowest depths to which 

 geological speculation ventured to penetrate. The intru- 

 sive cores of the Henry Mountains, on the other hand, are 

 represented by Mr. Gilbert as bounded on their under side 

 by a horizontal plane and as resting on horizontal strata. 

 They have, in fact according to him the shape of a huge 

 plano-convex lens, with its flat face downwards ; the 

 curved surface is however rather a portion of an oblate 

 spheroid than a sphere, for the trachytic masses are 

 somewhat flattened on the top ; some of them too are 

 oval rather than circular in plan. To an intrusive mass 

 of this shape he gives the name of a laccolite, from 

 XdxKof, a cistern, and \i8os, stone. 



His theory of the genesis of a laccolitic mountain is as 

 follows : — Lava was pumped up through a chimney or 

 fissure and at a certain point in its upward course spread 

 itself out between two adjoining beds in the form of an 

 intrusive sheet ; by farther additions of lava from below 

 the sheet is thickened, the overlying strata are more and 

 more arched, till at last they are bent up into a dome. 



Of course this involves the stretching of the overlying 

 strata ; in the case of one of the domes it is calculated 

 that there must have been an extension of 300 feet in 

 three miles. Mr. Gilbert has shown that this elongation 

 is rendered possible by the fact that at the time of their 

 flexure the beds were loaded by a crushing weight ; 

 directly the tension exceeded the limits of cohesion, and 

 a fissure was torn open, or rather directly a fissure would 

 have been torn open had the bending taken place at the 

 surface, the weight of the pile of strata overhead crushed 

 together the walls and closed the rent. That a cover of 

 rock, perhaps 7,000 feet, and possibly 11,000 feet in thick- 

 ness, would tend to this result is clear enough, but that it 

 did not always prevent rupture is shown by the numerous 

 dykes associated with the laccolites. Mr. Gilbert has 

 attempted to show by mathematical calculation that at a 

 given depth the overlying strata could not be lifted if the 

 area of the !accolite falls short of a certain value. His 

 method involves certain assumptions which render it 

 some'what unsatisfactory, and his conclusion seems to be 

 inconsistent with the explanation he gives of the formation 

 of a laccolitic mountain ; for according to him the first 

 step in that process is the production of an intrusive 

 sheet. This in itself involves the uplifting of the beds 

 above, and his calculations show that no uplifting could 

 take place till the sheet had reached a certain size. 



The failure, however, to solve by mathematical methods 

 a problem of this difficult nature by no means implies 

 a rejection of the theory. A much more important 

 matter is the examination of the evidence by which 

 the existence of these peculiarly shaped bodies of in- 

 trusive rock is supported. Mr. Gilbert has evidently 

 seen enough to satisfy himself on this point, and we 



