COMPOSITION AND PHYSICAL PHENOMENA OF TRAP ROCKS. 



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it forms the basis of all lavas, and of the greater number of trap rocks. Associated with 

 augite, and generally in a vitreous form, it constitutes some of the well-known modern 

 volcanic basalts, in which the greater or less preponderance of the latter mineral confers 

 the more or less black, dense, and ferruginous character which they so often assume. 

 Mixed with hornblende it forms a large class of ancient rocks, also called basalt when 

 the minerals are intimately blended, or greenstone when each is distinguishable. In 

 another condition, felspar, in a glassy but loosely aggregated state, composes a rock of 

 a rough, porous, and earthy aspect, called trachyte, which is found among both modern 

 and ancient lavas, and formations of still greater antiquity. In a compact state, the same 

 mineral is the base of many of the porphyries ; and in a more or less crystalline form, 

 associated with quartz, mica, and other minerals, it composes the great class of granitic 

 rocks. As felspar is not found in any of the aqueous sedimentary deposits, except in 

 a decomposed or regenerated state, it may therefore be considered the most characteristic 

 ingredient of all igneous rocks. There are, however, many other simple minerals which 

 combine to form these rocks ; but it is foreign to my present object to enter into further 

 details, it being sufficient to state, that from the above-mentioned prevalent types of 

 rocks, formed by heat, there are endless gradations into others, on many of which I 

 shall treat in the following chapters. 



2. Association and physical phenomena. — In taking a general view of the association and 

 physical phenomena of igneous rocks, let us first take a rapid glance at the method by 

 which the products of extinct volcanos 1 have been identified in their nature, with those 

 of volcanos in activity. 



Ninety years have nearly elapsed since two French academicians, collecting plants 

 among the hills of central France, were astonished by discovering numerous cavities, 

 resembling the craters of volcanos. From the lips of these cavities, currents of lava, 

 as fresh in aspect as if they had flowed yesterday, were traceable into the neighbouring 

 valleys, following their sinuosities, barring up their ancient water-channels, and moulding 

 themselves into the inequalities of the actual surface ; and to complete the analogy with 

 active volcanos, most of the mineral substances composing these lava currents were 

 found to be similar to those of Vesuvius and Etna. When M. Guettard, one of the 

 naturalists, first announced these discoveries, so unwilling were men of science to be- 

 lieve in phenomena, of which neither history nor tradition had preserved a record, that 

 scepticism long prevailed. The graphic and attractive descriptions of Montlosier 3 , the 

 mineralogical knowledge of Dolomieu, supported by the precise geographical delineations 



1 I might refer to a host of writers, both abroad and at home, including the early Italian writers, who have 

 proved trap rocks to be of volcanic origin, and in the following pages some of these will be adverted to. The 

 elementary works of Daubeny and Scrope must be consulted by those English readers who wish to comprehend 

 the chemical and mechanical phenomena of modern volcanos. 



2 Essai sur les Volcans d'Auvergne, Clermont, 1802. 



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