390 H. S. WASHINGTON IGNEOUS COMPLEX OF MAGNET COVE 



masses of tinguaite are mixed with them, it seems highly probable that 

 there was considerable motion in the outer portion, due probably to 

 convection currents. This was after differentiation and probably before 

 crystallization had set in to any great extent, since, while the two grade 

 into one another, the transition zone seems to be narrow, and leucite 

 crystals are not found in the foyaite, nor large ortbocla.se crystals in the 

 leucite- porphyry. 



RESUME OF EVIDENCE 



All the foregoing facts are in favor of the view that the area is prob- 

 ably a section of a laccolith, and, at any rate, that the main rock types 

 are differentiates in situ of one mass of magma. For convenience they 

 may be briefly recapitulated : 



1. The broadly elliptical shape of the area, surrounded by shales and 

 sandstones. 



2. The quaquaversal upturning and' the metamorphism of the con- 

 tiguous shales at many places along the border. 



3. The existence of a zone of what is apparently highly metamorphosed 

 shale along the highest parts of the ridge and elsewhere on the outer 

 slope, representing the remains of the original cover. 



4. A platy parting approximately parallel to the walls, which has been 

 developed in places in the foyaite. 



5. The serial arrangement of the various igneous rocks from the center 

 outward and concentric with the general border of the igneous area. 



6. The, on the whole, regular change in structure and size of grain 

 from the center to the periphery. 



A L TERN A TI VE H YPO THESES 



Williams' reasons for calling the syenites " dike rocks " are,* as far as 

 I can understand, the evidence of flow in their structure, as already 

 noted, and the porphyritic structure of the " leucite-syenites " and a tend- 

 ency to trachytoid structure in the foyaites. These characters are, 

 however, what we might expect to find in the rocks at the borders of 

 such a mass, since convection currents would be more likely to develop 

 here, and the border magma would naturally cool more quickly, and 

 hence give rise to such structural peculiarities. It must also be remem- 

 bered that " the more basic a magma the more granular and the coarser 

 its degree of crystallization." f 



* Williams : Op. cit., ]>. 277. 



f Pirsson : 18th Add. Kept. IT. S. Geol. Survey, part iii. 1898, p. 575 : cf. Iddings : 12th Add. Rept. 

 U. S. Geol. Survey, 1892, pp. 626 and 645. 



