Lavis — On the Structure of Rocks. 119 



demarkation between them and the intrusive granite, or syenite, 

 may be very ill-defined. Thirdly, the condition will be highly 

 favourable to contact metamorphism, which, in such cases, often 

 extends into the surrounding rocks for very considerable distances, 

 often many hundreds, or even thousands, of yards. 1 Jukes main- 

 tained that the granite forms the basis of many volcanoes, being 

 the source of the eruptive matter. It has been observed by Ootta, 

 that the smaller the dyke the smaller the grain, 2 which is ex- 

 plained by the more rapid cooling of the smaller mass. We have 

 the same in nearly all kinds of dykes, where the nearer we 

 approach the outer surface the finer the grain ; though volcanic 

 dykes in cones are an exception for some minerals. Negri and 

 Spreafico, in describing an expansion of porphyry near Lugano, 

 show that the felspars near the surface are invisible, so that the 

 rock is a euritic porphyry. Towards the centre of this great 

 mass the crystals are distinct, but round and imperfectly formed, 

 whilst in the dyke, which supplies this great mass, the crystals 

 are very perfect and large, often reaching three centimeters in 

 diameter. We see, therefore, that the perfection of crystallization, 

 and the type of resulting rock, are in direct relation with the 

 length of time and quietness of the cooling of the magma. In 

 the same way we may explain the crystals in the salbands of some 

 dykes, being smaller than those in the more central part. 



On the other hand, we may meet with various intrusive rocks 

 with more or less purely vitreous salbands, in which, in many cases, 

 the line of demarcation is often distinct and very sharp between 

 the dyke walls and the intrusive matter. There are also cases, as 

 in the dykes of liparite of the Ponza isles, which possess thick pitch- 

 stone salbands which are soldered to the walls of the quartzose 

 tufas. It would seem that the great resemblance of the two rocks 

 in chemical and mineralogical composition, and therefore the small 

 difference between their points of fusibility, a very slight excess of 

 temperature in the intrusive rock would be sufficient to fuse the 

 walls, and yet cool rapidly enough to prevent complete crystal- 

 lization, thus leaving the vitreous salband. This is aided, no 

 doubt, by the low heat conductivity of the surrounding tufas. 



1 L. Gatta, Vulcanismo, 1385, p. 28. 



2 Naumann, Lehrbuch cltr Geologie, 1858-1868. 



