March. 19, 1896] 



NATURE 



463 



time, as they pierce rocks of Cretaceous age. The Mos- 

 quito Range of central Colorado, and the Ten-Mile dis- 

 trict to the west of it, were studied by Jacob and Emmons, 

 and in these areas intrusive rocks of the same composition 

 occur as laccolites, sills, and dykes in more ancient rocks, 

 and even in the fundamental Archa-an complex. A few 

 (,ther cases are referred to. 



The study of these examples reveals the fact that the 

 theory has received several important accessions, and 

 even some modifications. The laccolite is beginning to 

 lose the familiar mushroom shape which, with its central 

 stalk, has done much to cause incredulity ; we find, in- 

 stead, excentric dykes and pipes, complex groups of 

 dykes, even sheets and dykes in suspicious proximity to 

 important faults, indicated as the feeders of the lens- 

 shaped masses of igneous material ; this gives much 

 more of structural verisimilitude to the type. Again the 

 masses are shown to be more often irregular or unsym- 

 nietrical in outline, faults as well as folds give easement 

 to the lifted cover, the so-called pine-tree laccolite, or 

 group of anastomosing sheets is not infrequent ; bunches 

 of laccolities irregularly intruded into yielding shales 

 take the place of the single "stone cistern," and the 

 laccolite is sometimes shown to cross the strata both 

 above and below. In addition to formal sections, many 

 of the beautiful outline andpanoramicsketches of Holmes 

 are reproduced ; these are drawn with such a wonderful 

 feeling for the run of the country, that the igneous structure 

 can often be made out from them alone. In fact, so 

 much has been made out of what material has been 

 already got together, that we feel bound to express a hope 

 that selected examples of toe group will shortly be mapped 

 out in detail by the skilled stratigraphers of the Survey, 

 in order to ascertain in such a favourably exposed region 

 what is the precise relation of laccolitic groups to the 

 folding of rocks, and the exact part played by them in 

 tectonic history. 



It is a remarkable fact that the rock in the laccolites 

 is almost uniform in character. It is called a porphyrite, 

 and contains phenocrysts of either hornblende, augite, 

 hypersthene, biotite, quartz, and felspar, or several of 

 these in a holocrystalline but compact matrix of quartz 

 and orthoclase. The silica percentage, omitting one 

 doubtful and exceptional type, varies from 69-55, but is 

 generally about 63, and the proportion of potash to soda, 

 2"5-4, is fairly constant. The porphyritic crystals, 

 chiefly intratelluric in origin, have increased in size after 

 the intrusion of the rock, and it is thought that the 

 phenocrysts of orthoclase, which occasionally occur, have 

 been completely formed after intrusion. Gilbert's hypo- 

 thesis, that the laccolitic structure is determined by con- 

 ditions of density, is not accepted in its entirety, the 

 author quoting with approval Dana's criticism that Gil- 

 bert's explanation "appears to be complete without 

 reference to this difference of density. With so powerful 

 a forced movement as the facts, if they are rightly inter- 

 preted, show to have existed, no other cause could be 

 needed for a flow to the surface in the case of an open 

 channel, or for a flow to any level in the strata at which 

 a fissure might terminate ; and this is true, whether the 

 lava be light or heavy." Hints are given throughout the 

 paper that orographic movements may often have had a 

 determining effect in the localisation of laccolites, a result 

 already reached by some observers in Britain. 



An exceptionally interesting piece of experimental work 

 is contained in Mr. B. Willis's " Mechanics of Appalachian 

 Structure." ^ A long series of experiments was under- 

 taken, and is here illustrated by twenty-one large plates, 

 while a number of maps, sections, and photographs from 

 the Appalachian ground are of use to compare with the 

 experimental results. In addition to the usual types of 

 folds defined by the opposed dips, we have a classification 



1 Thirteenth Annual Report of the Geological Survey of the United 

 States, 1891-92. (1893.) 



NO. 1377, VOL. 53] 



according to ccmpression into open, closed, and carinate 

 or isoclinal folds. In the Appalachians the following dis- 

 tricts are recognised : the district of (i) open folding, (2) 

 closed folding, (3) folding with faulting, (4) folding with 

 schistosity. One of the great questions to which the ob- 

 server is led is this : As the " conditions antecedent to 

 deformation were the result of sedimentation, does the 

 distribution of strata afford an answer to the questions 

 raised ?" To investigate this problem a mixture of bees- 

 wax with plaster of Paris and Venice turpentine of varying 

 consistency was used, in thin, large sheets, resting on a 

 plastic support, and covered by shot to act as a load. 

 This was contorted by lateral pressure applied by means 

 of a slow-motion screw. One of the first results to come 

 out was that any slight dip in the layers of material was 

 usually sufficient to initiate a fold. " In strata under load 

 an anticline arises along the line of initial dip, when a 

 thrust, sufficiently powerful to raise the load, is transmitted 

 by a competent stratum. The resulting anticline supports 

 the load as an arch, and, being adequate to that duty, it 

 may be called a competent structure." From this it 

 follows that the size of an anticline depends on the 

 competency of the stratum and inversely on the load. 

 Some evidence has been obtained in the field that the 

 initial dips in the Appalachians have influenced the fold- 

 ing. An interesting by-product of the experiments is the 

 fissuring of tissue-paper placed between the layers, along 

 lines at right angles to the wrinkles. The work then goes 

 on to consider the packing of folds and the formation of 

 faults. The plates illustrate the successive steps of each 

 experiment of which the details are thought worthy of 

 publication. 



Owing to the small scale of the maps and to the rapidity 

 with which surveying is carried out, it is often necessary 

 to generalise the geological as well as the topographical 

 features expressed on the maps, when it is desired to bring 

 out important tectonic characters. This is pointed out by 

 Prof Iddings, and atoned for by the conscientious com- 

 pleteness with which he has worked out the petrological 

 affinities of the intrusive and eruptive rocks of Electric 

 Peak and Sepulchre Mountain. ^ Separated now by a 

 great fault, with a downthrow to the north-east, along 

 which a valley has been excavated, one of these masses — 

 Sepulchre Mountain— was once situated almost directly 

 over the other. Sepulchre Mountain is made up of 

 volcanic accumulations, andesitic and rhyolitic breccias 

 and lava-flows, with dykes and other eruptive volcanic 

 products ; Electric Peak consists of intrusive sheets, 

 dykes, and " stocks " of porphyrite and diorite corre- 

 sponding in a general way in composition with the rocks 

 first mentioned, but differing in their coarser texture and 

 more plutonic aspect. The relative date of the members 

 of the intrusive series can be made out and paralleled 

 stage by stage with the volcanic products, so as to make 

 it clear that we have here side by side the surface and the 

 deep-seated products of one volcano. This is expressed 

 in tabular form, as follows : — 



Electric Peak. .Sepulchre Mountain, 



(rt) Intrusive sheets of per- (a) Andesitic breccias, 



phyrite. 



{b) Intrusion of dyke and 

 stock rocks in the following 

 order : — 



Pyroxene-porphyrite to 

 pyroxene- and horn- 

 blende-diorites. 

 1 lornblende - biotitedio- 



rites and porphyrites. 

 Quartz - biotite - diorite- 

 porphyrite. 



Such a parallelism cannot be the result of an accident, 

 and it affords a most useful opportunity for the study of 

 the relations of deep-seated and surface products ; of this 



1 Twelfth Annual Report of the Geological Survey of the United Stater, 

 1890-91. (1B92.) 



(3) Andesitic breccias and 

 dykes in the following 

 order :— 



Pyroxene-andesites to 

 pyroxene - hornblende- 

 andesites. 

 Hornblende - biotites-an- 



desites. 

 Dacites. 



