80 PEOF. J. W. JFDD ON TERTIARY 



of these rocks is, that the conditions of consolidation have variously 

 influenced the relative order of crystallization of the ferro-magnesian 

 silicates (pyroxenes and olivines) and the alumino-alkaline silicates 

 (felspars). In the gabhros, the mutual interferences of the several 

 minerals seem to indicate an almost simultaneous crystallization ; 

 but in all cases, so far as I have seen, the advantage in the matter 

 of priority seems to remain with the felspars ; for when crystalline 

 faces are developed they are always those of felspar individuals. 

 In the dolerites, and especially in those of the ophitic type, the prior 

 crystallization of felspars to both pyroxenes and olivines is a very 

 obvious feature. In the basalts it is often difficult to say whether 

 the one class of minerals has the advantage or not of the other as to 

 priority in crystallization ; but it is certain that skeleton- crystals 

 and microlites of both groups of minerals may be seen in course of 

 development, side by side, in the same glassy magma. 



XI. Alterations which the Rocks have undergone. 



In the facility with which their constituent minerals undergo 

 change, the basic rocks are perhaps only second to the ultra-basic 

 ones. Much of the confusion which has prevailed concerning the 

 gabbros seems to have arisen from the circumstance that, in the 

 majority of cases, it is the highly altered products of these rocks 

 which have been studied — materials like saussurite-gabbro, wurlit- 

 zite, and variolite, in which perhaps not one of the original 

 mineral constituents remains. In such rocks the felspars are found 

 changed into saussurite (zoisite &c.), the augite into hornblendes 

 (smaragdite, actinolite, &c.), and the olivine into magnetite and 

 serpentine. Still further changes may have taken place by which 

 the rock is converted into a hornblendic schist or gneiss. In the 

 same way the less perfectly crystalline varieties are found altered — 

 the dolerites into " diabases," the basalts into " melaphyres," " pala- 

 tinites," " basaltites," and finally into "wackes." Careful study under 

 the microscope often shows that the whole of the minerals which at 

 present make up the rocks are pseudomorphs of or derivatives from 

 the minerals of rocks similar to those described in this memoir. 



Although the highly altered igneous rocks are usually found in 

 connexion with the more ancient geological sediments, yet it would 

 be a mistake to infer that the amount of alteration which an igneous 

 rock has undergone is any criterion of its geological age. Some of 

 the Tertiary rocks in the district we are describing are found in 

 the most highly altered state; scoriaceous basalts are found con- 

 verted into crumbling amygdaloidal rocks, their cavities filled with 

 zeolites, chlorophseite, chalcedony, or calcite. Sometimes, indeed, the 

 whole is converted into a dark green clay or " wacke." Conversely, 

 we sometimes have the good fortune to find in the midst of a mass 

 of a very ancient igneous rock portions which have escaped the 

 extreme change, and in which the original and characteristic 

 minerals and rock-structures may be readily made out. 



The great value of the examples of the basic rocks in the district 



