Descriptive Arrangement of Volcanic Rocks. 31 



rieties being distinguished, according to the remaining pri- 

 mary characters, under the separate heads of — 



1. Texture. 3. Internal structure. 



2. Mineral disposition. 4. Natural division. 

 There is, however, one previous distinction which it is in- 

 cumbent to draw between the rocks of the trap family, and 

 which is rather of a geological than mineralogical nature ; 

 viz., their division into the two classes of lithoidal or mass- 

 ive, and fragmentary rocks ; according as they are compo- 

 sed of minerals intimately united by the force of crystalhne 

 aggregation ; or merely of separate parcels of matter, inco- 

 herent, or enveloped in a cement, whether crystalline or 

 earthy, but evidently of later origin than the fragmented 

 portion it encloses. 



The second step is to arrange the lithoidal rocks into ge- 

 nera, according to their broad general characters of mineral 

 constitution. Now it has been ascertained that all the rocks 

 of this family, with very few exceptions, are principally com- 

 posed of felspar and augite in varying proportions. The 

 felspar is sometimes partially or wholly replaced by leucite, 

 mehlite, olivine, or hauyne ; and this substitution is observed 

 only to occur when the proportion of augite in the rock is 

 very considerable. The augite appears in the same manner, 

 occasionally replaced to a greater or less extent by mica ; 

 and this occurs only when the felspar is greatly in excess. 

 The augite is either pyroxene or hornblende, the one seem- 

 ing to replace the other in proportion to the abundance of 

 felspar. Titaniferous iron and sphene are subordinate but 

 very general ingredients in these rocks ; the former is often 

 in considerable quantity, and abounds most in the augitic 

 species. Quartz occurs very rarely in crystals or grains ; 

 garnet, spinelle, sapphire, and other still rarer minerals can 

 only be reckoned as accidents. 



It is seen then that these rocks naturally group themselves 

 into two principal orders; viz. — 1. That in which felspar 

 predominates exceedingly. 2. That in which augite or the 

 ferruginous minerals are in excess ; or at least so abundant 

 as to stamp their character on the rock. This is, in fact, the 

 division which has been generally made by the continental 

 geologists, who have called the former order trachyte, the 

 latter basalt. 



But the shades of mineral composition amongst the com- 

 pound rocks of this family are so varied in nature, and grad- 



