40 



period in which theïr ingredients retain a viscid consistency, will , from the éxpansion con- 

 sequent on crystallization § be exposed not only to mutual lateral pressure but to pressure 

 from beneath , and this will tend , according to the rate of refrigeration and other circum- 

 stances, more or less to obscure or even obliterate the spherical form. Where there is a 

 considerable proportion of mica the concentric laminar arrangement will still be preserved. 

 I cannot follow out this view here; but the experiment on the gradual cooling of molten 

 rock first made by Mr. Gregory Watt, and frequently repeated since , would seem to ex- 

 plain all the gradations of igneous rocks. With reference to the hypogene rocks, in 

 order to conceive how the different species may be produced from igneous fluid beneath the 

 earth's crust, we need only consider under what different conditions the fluid must have 

 existed as to tranquillity , agitation, or motiou , — the extent and form of refrigerating ' 

 surfaces , — the nature , thickness , and pressure of the refrigerating masses themselves, 

 whether rock, or sea, or both, — the rate of diminution of temperature, — and the propor- 

 tions of chemical ingredients. Many of these conditions may have varied in different por- 

 tions of the same grcat fluid mass , and at different stages in the process of its crystalliza- 

 tion and solidification. The phenomena attending Mr. Watt's experiment even seem to shew 

 that the globular, cuboidal, or prismatic concretionary structure of micaceous granite might 

 pass into the laminar gneissose structure merely through oscillations in temperature duriüg 

 solidification. An analogous passage from nodules into layers , and , in the latter , the 

 arrangement of the crystals of one of two or more different minerals in continuous parallel 

 laminae, characterises some volcanic rocks. 



If éxpansion on crystallization be attributable to the nether hypogene fluid masses, and 

 they are gradually solidified from above downwards , a slow upheaval of the superincumbent 

 crust must attend their solidification when they are of sufficiently great extent. This may 

 be the cause of the elevation of Scandinavia and other countries, at present in progress. 



Whatever may be the origin of the bedded structure of the hypogene rocks, it is to a 

 similar stratification , combined with the vertical joints , that the forms assumed by the ex- 

 posed masses on Pulo Vhin must probably be referred , and to these I now finally return. 



The blocks protruding from the hills or ranged along the shores of Pulo Vhin are more 

 solid and less decomposable masses and nuclei, of which the forms, and the directions of the 

 sides and axes, have, in almost every instance, been determined by structural planes, and 

 which remain after the surrounding rocks have disintegrated and been washed away. With 

 respect to the latter , it is obvious that while the Island has been extending by the growth 

 of alluvium in its bays , its more open coast has been slowly retreating , so that what 

 was once a part of the solid land is now a band on its border washed by the sea , but still 

 exhibiting numerous rocky remnants. The larger masses still evidently occupy their original 

 positions. Frequently their seaward face is curved. Sometimes another mass stands behind 

 merely separated from that in front by a chasm whose sides are parallel. With respect 

 to the decomposition of the rocks on the hills, the soil is entirely derived from this source 

 with the exception of a very slight superficial mixture of vegetable matter, which in many 

 places is absent. In general , however, the blocks that remain are decomposing with exceed- 

 ing slowness. One exception I noticed in the NW. — SE. side of the rock described at 



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