296 



SCIENCE. 



[Vol,. II., No. 30. 



tive agencies to make a continent out of a series of 

 arcliipelagos. 



'i"liere are two points requiring explanation in this 

 connection, — first, the supposed deeply seated locali- 

 ties wliere granite is produced; and, second, the ori- 

 gin of foliation in the schists. 



We should naturally expect that the earlier igneous 

 rocks would have been derived from reservoirs quite 

 near the surface, because of the thinness of the crust. 

 With this notion agrees the presence of cavities con- 

 taining liquid, and of hydrated minerals, which are 

 more common in the older eruptive rocks, and have 

 led to the aqueo-igneous theories of the origin of 

 granile. Water would be scarce at great depths, and 

 hence these rocks ought to originate near the surface 

 where moisture was abundant. It seems to us that 

 this consideration should more than balance the argu- 

 ments usually cited in favor of the origin of granite 

 at enormous depths, as it is difficult to see how both 

 can lie true. 



Mr. 11. C. Sorby has led the way in studies of the 

 mineral constituents of eruptive rocks. He measures 

 the included cavitie-; in the component minerals, and 

 calculates how much the contained substances must 

 have contracted in cooling, allowing for an increase 

 in the temperature of the point of vaporization under 

 pressure. By assuming the temperature to be cor- 

 rectly determined, he ascertains the amount of press- 

 <ire indicated by mathematical formulae, and finds 

 it to be the equivalent of a thickness of 40,000 feet 

 of overlying rnck in Cornish granites, and of 60,000 

 feet in Scotch granites. Later writers seem to have 

 regarded this pressiu-e as certainly produced in the 

 way thus suggested, and that its appearance at 

 tlie surface has been due to an enormous erosion 

 which has denuded the overlying blanket. This 

 cimcliision is not necessary; for, 1°, an enormous 

 pressure would result from the tangential force of 

 contr^iction, which would be entirely adequate to 

 liave produced the cavities. 2°. The necessity of an 

 erosion of 40,000 feet over all the granites in every 

 part of the world cannot be maintained. In North 

 America, for example, it would necessiiate the sup- 

 positi'in that nearly eight miles' thickness of rock 

 liad been removed from one-fourth of the surface 

 since the Liuiientian, for the blanket removed would 

 have equalled in dimensions the crystalline areas. 

 Till', mere statement of the amount of denudation re- 

 quired refutes the theory. 3°. By reference to existing 

 volcanoes, it is plain that a column of lava will often 

 be adequate to exert the needed pressure. Teneriffe 

 rises 12,000 feet above the ocean, and its cone de- 

 scends 18,000 feet more to the submarine plateau. 

 When the crater is full of melted lava, there must be 

 a pressure of 3f',000 feet at the base of the cone: 

 hence the lava from the reservoir supplying Teneriffe 

 might exhibit the vacuities produced by a pressure of 

 30,000 feet without any weight above the peak. 



When molten lava pours down the side of a crater, 

 the included vapors and liquids must disappear he- 

 cause of the removal of the pressure; but, after a sub- 

 sianlial crust has formed, the peculiar markings 

 imprinted at the great depth would remain: lience 



we can understand how it is that the vacuities are to 

 he seen both in granites and lavas that have been sub- 

 jected to great pressure. At the Boston meeting of 

 this association I endeavored to show that there are 

 mountain masses of granite in New England possess- 

 ing all the physical characteristics of volcanic cones. 

 The material must have been liquid, hot, ejected 

 from a vent, and flowed over a plateau, building up a 

 cone, and indurating the underlying floor. It was 

 claimed that such phenomena could be explained 

 only by supposing the granite to have been erupted 

 just like lava. This granite contained the usual 

 vacuities indicative of great pressure just as they 

 are also found in the lava of Monte Somma or the 

 trachyte of Ponza. 



Wlien one examines the interior structure of mod- 

 ern lava-flows, he is surprised to find beds nearly as 

 well defined as the foliation of schists. Around 

 vents like Vesuvius or Etna the lava accunuilatcs 

 naturally in quaquaversal sheets, no one eruption 

 being very extensive. When steam and hot water 

 are copiously supplied from the caldron, there may. 

 be flows of hot mud and tufa. The closing phases 

 of eruptions are usually showers of ashes falling 

 upon the cone or beyond. If the vent is beneath the 

 ocean-level, the lava is minutely subdivided and the 

 deposit will be like sand or gravel. Between the ig- 

 neous flows the ordinary aqueous agencies will wear 

 off excrescences, and scatter the fragments down the 

 slope. These various agencies will produce a con- 

 centric stratiform arrangement in the whole mass. 

 Where the eruption is massive, a similar set of layers 

 will be formed. 



This mass of volcanic material will bo very sus- 

 ceptible to metamorphic influences when ])laced 

 under the proper conditions of heat aiul pressure. 

 As the result, new minerals will be formed, arranged 

 in foliated beds or schists. Thus briefly stated. may 

 be the origiji of foliation. So hmg ago as 182."), 

 I'oulett Scrope advocated essentially this doctrine for 

 the arrangement of the crystalline particles in the 

 crystalline schists, having found an analogous struc- 

 ture in certain volcanic accunuilations. 



Sufticient has now been said in advocacy of our 

 doctrine that the first land consisted of volcanic 

 islands. This was the Laurentian or azoic accumu- 

 lation. Cartographers have not yet distinguished the 

 several crystalline deposits, so that it will not be 

 practicable at present to point out the supposed vol- 

 canic areas of the Hastings, Grenville, Montalban, 

 Huronian, and other eozoic periods. Sedimentation 

 would also act so that in this age many beds must be 

 referred to an aqueous derivation. By the close of 

 the eozoic the continent was outlined; or at least the 

 framework of the future superstructure was put into 

 position. The broader patches about to be men- 

 tioned had their origin in the earlier numerous is- 

 lands cemented by detrital accumulations. 



The more important areas developed in the eozoic 

 must have been Greenland, Canada east of Lake 

 Winnipeg, the Atlantic district, the Rocky Moun- 

 tains, the Sierra Nevadas, and numerous butles over 

 tlie Cordilleras. The three great depressions of Hud- 



