CHAPTER V 



TRAP ROCKS. 



Trap Rocks compared with Volcanic Products, and shown to be of Igneous origin. 

 Distinctions between contemporaneous and intrusive Trap Rocks. 



Hitherto we have exclusively considered the nature of sedimentary deposits, but 

 our account of them can no longer be advantageously continued, without occasionally 

 describing rocks of very dissimilar characters, which have been abundantly intruded 

 amid the strata. Already indeed the fragments of some of these rocks have been 

 alluded to as forming part of certain conglomerates in the New Red System. These 

 are the basalts, greenstones, porphyries, and syenites, which constitute a portion of 

 the Trap Rocks of geologists. As my readers who have not studied geology will find 

 in the sequel, that many important inferences in the theory and practice of the science, 

 depend on the right understanding of the phenomena connected with these rocks, I 

 deem it advisable in this introductory chapter, to explain their analogies with the 

 products of existing volcanoes, and to point out how they have been proved to be of 

 igneous origin. A slight acquaintance with volcanic phenomena teaches us, that 

 they are the results of some general and deeply seated cause, which occasions eruptions 

 of gaseous and earthy matters, or of lava, at irregular intervals, both under the atmo- 

 sphere and beneath the ocean. Hence volcanic products are naturally divisible into two 

 great classes ; sub-aerial, and sub- aqueous. The first, being in many respects open to 

 our investigation, is to a considerable extent understood. The second, for the most 

 part hidden from examination, is necessarily but little known, though recent obser- 

 vations have thrown some light upon it. It is to the last-mentioned of these classes, 

 or submarine volcanos, that perhaps all our British trap rocks are referable. I will first 

 very briefly point out the resemblance in mineral characters, between these rocks and 

 modern volcanic productions, and secondly will endeavour to show, that the two classes 

 are intimately connected with each other, both by association and physical phenomena. 



1 . Mineral character, fyc. The most common ingredient in modern and ancient 

 lavas, as well as in trap and granitic rocks, is felspar. This mineral assumes a consider- 

 able variety of forms, which differ so greatly from each other, that a novice finds it dif- 

 ficult to recognise in them the same substance. In an earthy, vitreous, or compact state, 



