

I 



on the Lake Superior Land District. 231 



Isle Royale. Along the lines of these fissures existed numerous 

 volcanic vents, like those observed at this day in Pern, Guate- 

 mala, and Java, which were characterized by periods of activity 

 and repose. From these vents were poured forth numerous 

 sheets of trap, which flowed over the sands and clays then in 

 the progress of accumulation. During the throes and convul- 

 sions of the mass, portions of rock would become detached, 

 and rounded simply by the efl'ects of attrition, and jets of melted 

 matter be projected as volcanic bombs through the air or water, 

 which, on cooling, would assume spheroidal forms; while other 

 portions of the rock, in a state of minute mechanical division, 

 would be ejected in the form of ashes and sand, which, mingling 

 with the water, would be deposited, as the oscillations subsided, 

 among the sands and pebbles at the bottom of the sea. During 

 the whole of this period of volcanic activity, the sands which 

 now form the base of the Silurian system were in the progress 

 of accumulation, and became mirigled with these igneous pro- 

 ducts. The level of the sea, as evidenced by the ripple-marks, 

 was subject to repeated alterations: sometimes it rose so shoal 

 that the marks of the rippling waves were impressed on the 

 sands; at others, it sank to unfathomable depths. 



In the process of consolidation, the rocks became traversed by 

 numerous fissures, and the water, charged with lime, was forced 

 in like jets of steam, filling them with materials different from 

 the enclosing mass. In this way the pores in the conglomerate 

 and the vesicles of the amygdaloid were filled. 



Thus alternating bands of igneous and aqueous rocks were 

 formed, until finally the great crystalline masses of greenstone 

 were protruded through the fissures, not in a liquid, but in a plas- 

 tic state, hfting up the bedded trap and conglomerate, and caus- 

 ing them to dip at high angles from the axis of elevation. As 

 the volcanic action diminished in energy, the detrital rocks en- 

 closed fewer igneous products; and, when it ceased altogether, 

 sand and clay, derived from regions remote from the lines of dis- 

 turbance, were the only materials which, for a time, were depos- 

 ited on the floor of the ocean. 



To illustrate the nature of volcanic action, w^e need only to 

 revert to instances which have hapj)ened within the present cen- 

 tury. So recently as 1831, a mass of rock rose up from the sea 

 ^ear the coast of Sicily, where soundings had previously been 

 JT^ade to the depth of six hundred feet. This mass, which was 

 subsequently known as Graham's island, rose gradually from the 

 ^ater, until it attained an elevation o( two hundred feet above 

 the surface, and a circumference of three miles. It slowly di- 

 i^mished to the circumference of three hundred yards, and in 

 the course of three months sank eleven feet below the water, 

 leaving a dangerous reef 



f 



