656 



GEOLOGY. 



blende, mica, titaniferous iron, and occasionally augite. The name is derived from rpa- 

 Xve, rough, and expresses the harshness of the substance to the touch. Trachyte is of a 

 whitish or grey colour, and appears to have been an abundant product of volcanoes 

 during the tertiary period, which have continued to the present to erupt trachytic lava. 

 It occurs largely in Hungary and Auvergne, and in vast quantities in South America, 

 some of the loftiest heights of the Cordilleras being composed of it. The products of 

 extinct volcanoes are entirely identical with those that are now active, and so strikingly 

 resemble the older basalts and the other Plutonic rocks in general, that the common origin 

 of the whole, modified by different circumstances, may be inferred. 



There are no examples of igneous formation in process now within the bounds of our 

 own island ; but the granitic and trappean groups "are abundantly disseminated, and bear 

 witness to the fiery activity of former periods. Playfair estimated the granite in Scotland 

 to occupy an area of 940 geographical square miles, equal to 1250 English miles, or about 

 a twenty-fifth of the country ; but a more recent estimate gives to the granite of North 

 Britain an area of 1760 miles. In' England it is supposed to be under 300 miles, or 2000 

 in all ; while the trap in both parts of the kingdom is not less than 3000 square miles, 

 making the whole extent of the igneous formations in Great Britain amount to 5000 miles ; 

 about one seventeenth of the whole surface, the larger proportion of which belongs to 

 Scotland. We must look for the origin of the Plutonic rocks to those interior regions of 

 the globe from whence the igneous products of the present era are erupted ; but while the 

 formation of the different groups has transpired from similar causes, it has taken place 

 under different circumstances. The granites, excepting the most ancient, have been 

 erupted, cooled, and solidified, under the pressure of superincumbent strata ; the traps 

 have undergone the same process, under the depth and pressure of the ocean ; and the 

 volcanic rocks have refrigerated and become solid masses at or near the surface, in contact 

 with the atmosphere. These diverse conditions will explain many diversities of structure 

 in these productions of the interior fires ; and, as involving a different rate of cooling, 

 they may account for the remarkable distinction between granitic veins and porphyritic 

 and basaltic dykes, which has given rise to so muah speculation. Granite and its varieties 

 are found intruding into the minute fissures of the stratified masses, producing small 

 diverging branches ; while the trappean rocks generally pass through them without these 

 ramifications. It seems likely that the circumstances under which the trap erupted 

 admitted of a more rapid cooling and reduction to solidity than what obtained in the 

 case of the granite. The latter retaining its fluidity longer, penetrated into the small rents 

 of the strata, which the former, for the reason assigned, had not the capacity to do. 



The Needle Rocks, Isle oi Wight. 



