no A T TOFSCIENCES1 POWELL'S SURVEY 83 



ments when thus brought together are clear enough, even though they do not always mention 

 the attitude of the underlying strata; but in the original text they are scattered and almost lost 

 among other matters. Moreover, in the published account of two other well-exposed laccohths 

 the underlying strata are not mentioned: The Marvine, the northernmost of all, "stands forth 

 on a pedestal, devoid of talus, naked and alone" (42) ; and as to the Scrope, northeast of Pennell, 

 "the erosion of its matrix has left it a conspicuous crag" (47). Even in the generalized struc- 

 tural summary above cited, the underlying strata are not given prominence; for besides the 20 

 instances in which erosion is not yet deep enough to reveal the base of the laccolith, and the five 

 instances in which one side of a laccolith has been eroded, "exposing the core of trachyte to its 

 base and showing undisturbed strata beneath it," there are seven other instances which are next 

 noted as having suffered a still greater erosion, but in connection with which nothing is said of 

 the underlying strata. Silence here can not mean that the fundamental strata were not seen, 

 but that they were so plainly seen as to have been taken as a matter of course. 



There can therefore be no question that Gilbert determined the general facts of laccolithic 

 structure correctly. There may, however, be question whether certain igneous masses which 

 have been in later years explained as laccoliths, after the laccolith idea had been generally 

 accepted, are truly of that nature. For example, Hauthal describes two bold mountains, Payne 

 and Fitzroy, in the southern Andes as laccoliths, and explicitly states that they have a partly 

 eroded cover of Cretaceous strata; but he does not give explicit account of any underlying 

 undisturbed strata. 5 It is quite possible that the conception of laccoliths has become so popular 

 that masses of other nature have been called laccohths without sufficient assurance that they 

 have a level base as well as a domed top. 



5 Mitteilungen iiber den heutigen Stand der geologiscben Erforschung Argentiniens. 9th Internal. Geol. Congress, Vienna, (1904), 1905, 

 649-«56. 



