2O Igneous Rocks. 



structure were formed by igneous agency, and this in 

 ancient cases is not the less certain though the vesicles- 

 have since been filled by the infiltration and deposition of 

 mineral matters in solution, such as carbonate of 

 lime, zeolites and silica. Such igneous rocks are called 

 amygdaloids, and it has not infrequently happened that 

 on the surfaces of old masses of rock, the amygdaloidal 

 kernels, say of carbonate of lime, have been dissolved 

 out by the influence of rain-water bearing carbonic 

 acid, and the surface has regained its original vesicular 

 appearance. 



Experience also tells us that some modern lavas are 

 crystalline that is to say, in cooling, their constituents, 

 according to their chemical affinities, have crystallised 

 in distinct minerals such as augite, various felspars, &c. 

 When we meet with similar, even though not identical 

 crystalline rocks, such as felspar porphyries, trachytes, 

 diorites and dolorites, associated with old strata, we are 

 therefore entitled to consider them as having had an 

 igneous origin. 



In modern volcanic regions, such as Iceland, and in 

 tertiary regions dotted with extinct volcanoes of Mio- 

 cene or later age, where the forms of the craters still 

 remain, the lavas are often columnar; and when we 

 meet with columnar and crystalline rock-masses of 

 Silurian, Carboniferous, or of any other geological age, 

 we may fairly assume that such rocks are of igneous 

 origin. Modern lavas have often a vitreous structure 

 (glassy) such as obsidian, which its ancient analogue 

 pitchstone closely resembles. Others possess a slaggy 

 structure, and are sometimes formed of wavy ribboned 

 layers that indicate a state of viscous flowing, similar 

 to the contorted ribbon-like structure common in iron 

 and other slags. Iron slag in fact is nothing but arti- 



