PHYSICAL PBOPEETIES OF VOLCANIC EOCKS. 87 



crystalline" texture of ordinary lavas, and yet it may not have been 

 erupted or subjected to that mechanical action which is the most con- 

 spicuous feature of volcanism. It may have been intruded into a dike, or 

 between strata, and only brought to daylight after the lapse of many 

 geological periods by the agency of denudation. Many of the quartz 

 porphyries and the intrusive or "laccolitic" trachytes of the West, and 

 many basalts or dolerites, are of this character. Are these truly volcanic 

 rocks! Before attempting to answer this inquiry let us advert to the 

 wholly crystalline rocks, such as granite, syenite, diorite, diabase, &c. 

 These are not usually accounted to be volcanic rocks ; yet they have been 

 heated and rendered plastic, and they have been intruded into narrow 

 dikes and veins and between strata, though they have never been erupted, 

 so far as we know. Between the intrusive rocks of a wholly crystalline 

 texture and the intrusive rocks of a half- crystalline texture there may be 

 found a true transition of varieties, and a hard and fast line cannot be drawn 

 between them. Chemically, the two classes are sensibly exact counterparts 

 of each other, and are very nearly so in respect to their constituent min- 

 erals. But the failure to find a boundary is no bar to classification, which 

 takes account not only of differences but also of affinities; and hence, while 

 speaking of volcanic and granitoid rocks as distinct classes, we must still 

 keep in mind the reservation that there is a border country between them. 

 Having indicated the characters which belong to all volcanic rocks as a 

 class, and which at the same time serve to distinguish them from other classes, 

 we may next proceed to consider how they differ among themselves, and 

 what affinities exist between the different groups. It may be repeated here 

 that considerations relating to the genesis of rocks — the causes and pro- 

 cesses which have made them what they are — should not be directly or 

 primarily taken into the account. We know too little about their genesis, 

 and any attempt to include such considerations would merely lead us to 

 embody what we conjecture rather than what we know, and would almost 

 certainly mislead us. We can take account only of well-known facts, and 

 these are to be found chiefly in those chemical and physical characters 

 which have been extensively studied and compared. These are chiefly as 



